My group started out with several people, but has dwindled to two players and the DM. My character is a Rogue, and the other player character is a Ranger. The Ranger's companion is a mountain lion, who deals a lot of damage and does well as a front-line, but I also decided that my character needs to be more front-and-center as well to take the heat off the Ranger and give our party more balance. At first our characters were built more for story rather than tactics (also when we first started we had no idea what we were doing), and now we're having to think more tactically because of our smaller party. I have made a second character (Fighter/Gunslinger) for this campaign that is off to the side to bring along should the need arise, but the Rogue is my main.

So, on top of my eight levels in Rogue/Thief, I have three levels in Fighter/Eldritch Knight.

My cantrips are: Booming Blade and Shocking Grasp.

My spells are: Absorb Elements, Shield, and Thunderwave.

My ability scores are:

• Strength: 12
• Dexterity: 19
• Constitution: 12
• Intelligence: 10
• Wisdom: 14
• Charisma: 15

Feat: Luck

So here are my thoughts: I am definitely thinking of upping my constitution, or my strength scores. And I'm wondering if I should really focus on the Fighter class more, or keep going with Rogue since I am already at level 8 with Rogue and would be losing out on something regardless. I'll be talking with my DM about this as well, but I'm interested in seeing what other people's opinions are! Other than my aim to balance the party, I enjoy doing a lot of damage, which is why I decided on playing a Rogue in the first place. What class should I start focusing on, what scores do I need to improve, what spells should I consider, and if there any other feats I should add on. Any advice or general insight would be great!

I don't think you should spend any focus on your strength. You need a finesse weapon for dealing sneak attack damage, anyways, and for those, you can use dex. A +1 to dex would be top on my list. Maybe look into feats that give you a +1 to dex, as well as something extra.

If you are keen on getting more hit points, the feat tough will give you more value for the ASI.

Strength isn't really necessary for you if you are using a finesse weapon like a rapier. Getting tough as a feat would be more immediately beneficial in regards to increasing your hit point total then just increasing you Con mod by +1. 

If you are wanting more damage potential, going more into fighter might be good. For one, the fighter hit die means you get more hp every level then you would as a rogue on average. then the class features give yo more combat options. Not to mention more spells.

1. At level 5 you get the Extra Attack feature,  you can attack twice, instead of once, whenever you take the Attack action on your turn.
2. At level 7 you get the War Magic feature,  when you use your action to cast a cantrip, you can make one weapon attack as a bonus action. 
3. At level 10 you get the Eldritch Strike, When you hit a creature with a weapon attack, that creature has disadvantage on the next saving throw it makes against a spell you cast before the end of your next turn.

And at 4th, 6th, and 8th level of fighter you get the change to increase your ASI or get a feat. You could choose to increase your Con modifier 3 times or pick feats that might benefit you.

1. Tough, as mentioned above.
2. Defensive Duelist, When you are wielding a finesse weapon with which you are proficient and another creature hits you with a melee attack, you can use your reaction to add your proficiency bonus to your AC for that attack, potentially causing the attack to miss you. This might help you avoid a few big hits in combat.
3. Mobile, You are exceptionally speedy and agile. You gain the following benefits: Your speed increases by 10 feet. When you use the Dash action, difficult terrain doesn't cost you extra movement on that turn. When you make a melee attack against a creature, you don't provoke opportunity attacks from that creature for the rest of the turn, whether you hit or not. This would be good for a more hit and run style.

These are a few decent options I think. I hope this helps some.

I think that STR is wasteful.  I'd definitely not go that route.  I'd get your dex to 20, maybe via a half feat, and then start pumping con probably.  I'm really torn at how I'd go about increasing this character, I'm generally not a fan of the thief archetype and you're 8 levels deep into it.  On one hand, going more fighter to attack more often is good, but you sacrifice your improving your sneak attack dice. 

Your cantrips aren't really helping you though if you go deeper into fighter; you're better off spamming booming blade until fighter 7 (character level 15) when warmagic kicks in, then you'll booming blade anyways, plus a basic attack.  Shocking grasp is not helpful at all since you can't sneak attack with it, and you can bonus action disengage anyways.  My gut tells me you're better off being the best rogue you can be, and leaving EK at 3.

Quote from account256 >>

I don't think you should spend any focus on your strength. You need a finesse weapon for dealing sneak attack damage, anyways, and for those, you can use dex. A +1 to dex would be top on my list. Maybe look into feats that give you a +1 to dex, as well as something extra.

If you are keen on getting more hit points, the feat tough will give you more value for the ASI.

Quote from RoughCoronet0 >>

Strength isn't really necessary for you if you are using a finesse weapon like a rapier. Getting tough as a feat would be more immediately beneficial in regards to increasing your hit point total then just increasing you Con mod by +1. 

If you are wanting more damage potential, going more into fighter might be good. For one, the fighter hit die means you get more hp every level then you would as a rogue on average. then the class features give yo more combat options. Not to mention more spells.

1. At level 5 you get the Extra Attack feature,  you can attack twice, instead of once, whenever you take the Attack action on your turn.
2. At level 7 you get the War Magic feature,  when you use your action to cast a cantrip, you can make one weapon attack as a bonus action. 
3. At level 10 you get the Eldritch Strike, When you hit a creature with a weapon attack, that creature has disadvantage on the next saving throw it makes against a spell you cast before the end of your next turn.

And at 4th, 6th, and 8th level of fighter you get the change to increase your ASI or get a feat. You could choose to increase your Con modifier 3 times or pick feats that might benefit you.

1. Tough, as mentioned above.
2. Defensive Duelist, When you are wielding a finesse weapon with which you are proficient and another creature hits you with a melee attack, you can use your reaction to add your proficiency bonus to your AC for that attack, potentially causing the attack to miss you. This might help you avoid a few big hits in combat.
3. Mobile, You are exceptionally speedy and agile. You gain the following benefits: Your speed increases by 10 feet. When you use the Dash action, difficult terrain doesn't cost you extra movement on that turn. When you make a melee attack against a creature, you don't provoke opportunity attacks from that creature for the rest of the turn, whether you hit or not. This would be good for a more hit and run style.

These are a few decent options I think. I hope this helps some.

Quote from crzyhawk >>

I think that STR is wasteful.  I'd definitely not go that route.  I'd get your dex to 20, maybe via a half feat, and then start pumping con probably.  I'm really torn at how I'd go about increasing this character, I'm generally not a fan of the thief archetype and you're 8 levels deep into it.  On one hand, going more fighter to attack more often is good, but you sacrifice your improving your sneak attack dice. 

Your cantrips aren't really helping you though if you go deeper into fighter; you're better off spamming booming blade until fighter 7 (character level 15) when warmagic kicks in, then you'll booming blade anyways, plus a basic attack.  Shocking grasp is not helpful at all since you can't sneak attack with it, and you can bonus action disengage anyways.  My gut tells me you're better off being the best rogue you can be, and leaving EK at 3.

Okay, so what I see so far...

Strength unnecessary: heard!
Upping Dex to 20: heard! Gonna definitely do that.
Not a fan of the Thief Archetype: VERY MUCH SAME. I picked it before I knew what I was doing and what I preferred, and playing as a thief hasn't been particularly useful except Second Story Work has gotten me out of some sticky situations. But now we have a Flying Carpet and an Immovable Rod, so ¯\_(ツ)_/¯ I've found Thief to be very situational, and weak.
The Tough Feat: I've been looking at this one, as well as the Mobility feat, and I think they're a good idea to take.
Sneak Attack: I think this is definitely something to consider, so thanks for bringing that up! 

I am thinking that since I have so many levels in Rogue, it may be best to continue that route because my Sneak Attack dice will be going wayyyy up. But the hit dice for Fighter, as well as stronger spells is always a bonus - I just won't be getting as far with this class. At least through my first character, I'm really learning how to balance things out haha

I really appreciate y'all stopping by and giving me this advice! It's definitely narrowed things down for me and given me a clearer idea of what I'd like to do. :)

Thief as an archetype can be fun but it's really dependent upon the player using the strengths of the class; IE fast hands with the healer feat, using it to poison arrows, throw caltrops, etc.  Also 2nd story work can be really fun as well if you've got things to climb and parkour over. 

At higher levels the thief gets THE BEST level 13 and level 17 rogue abilities, but most games don't go that high. 

Also, unless your character is the face of the party your CHA should be swapped with your INT.  Eldritch knights are INT casters.  Also, some of the rogue's most useful abilities are INT skills (Investigation, arcana).

The shocking grasp cantrip is kind of a waste (especially with only 10 INT) because as a rogue you get bonus action disengage.  Something like green flame blade would be useful to give you fire damage on an attack.  Plus, as rogue you can bonus action disengage.  Shocking grasp is a way of robbing an enemy of their reaction to run away.  You're way better off using booming blade then bonus action disengage to get out.

Defensive duelist is a trap feat, especially if you have the shield spell.  Yes, it's resource free, but it only works against a single attack that turn.  The shield spell gives you the AC boost until the beginning of your next turn. 

What level is your campaign running to? 

If I were you because of the nature of your characters and the fact you had built them up for a full party and not just 2 players I'd ask your DM if you would be willing to re-roll your character or at least take another subclass of rogue.

Quote from Ogelthorpe >>

Thief as an archetype can be fun but it's really dependent upon the player using the strengths of the class; IE fast hands with the healer feat, using it to poison arrows, throw caltrops, etc.  Also 2nd story work can be really fun as well if you've got things to climb and parkour over. 

At higher levels the thief gets THE BEST level 13 and level 17 rogue abilities, but most games don't go that high. 

Also, unless your character is the face of the party your CHA should be swapped with your INT.  Eldritch knights are INT casters.  Also, some of the rogue's most useful abilities are INT skills (Investigation, arcana).

The shocking grasp cantrip is kind of a waste (especially with only 10 INT) because as a rogue you get bonus action disengage.  Something like green flame blade would be useful to give you fire damage on an attack.  Plus, as rogue you can bonus action disengage.  Shocking grasp is a way of robbing an enemy of their reaction to run away.  You're way better off using booming blade then bonus action disengage to get out.

Defensive duelist is a trap feat, especially if you have the shield spell.  Yes, it's resource free, but it only works against a single attack that turn.  The shield spell gives you the AC boost until the beginning of your next turn. 

What level is your campaign running to? 

I think thief as an archetype is going to perform better in some campaigns than others.  If you're doing an urban intrigue campaign skulking around, infiltrating people's houses and such secret agent style, thief's probably a pretty solid pick.  If you're out in the woods, trying to rescue damsels from dragon lairs or orc camps, I am not certain that thief brings a lot to the table.

I agree with pumping int as an EK, I hadn't noticed his int before.  That makes shocking grasp even worse.  Now, a low int on an EK isn't that big of a deal but it /does/ limit your options on your magic.  For example, booming blade doesn't really care about his low int.  Something like light would be super useful since he's a human, and light doesn't care about his int.  Shield and Absorb elements don't care about his int.  Thunderwave...I might not take that one because it cares for int, but even on a save it does half damage so, maybe I'd keep it.  I would certainly look at the int requirements while picking spells moving forward.  ABJ are probably going to be good to go, but EVO are probably going to be iffy in general.

Agree on defensive duelist.  That's bad and made worse with shield access, especially at higher levels when things start to multi-attack you.  If it was a flat bonus to all attacks, then I would like it more.