I'm in a bit of a bind here, and I hope some of you more experienced DMs may have some good input.

I'm the DM for a campaign with six players, delving into the secrets of an old city and generally being PCs. Most of the group is new to the game - A Rogue, two Bards (ohgodno), a Grave Cleric, a Monk and a Barbarian. The last two know the game and have played before. Recently, I got into talks with the Barbarian to maybe homebrew a cool sub-class for him.

He plays a Barbarian Warforged, fluffed as a recently-awakened killer-bot that was once used in the old realm thousands of years ago, of which he knows little about. His Rage is fluffed as him going into "kill-mode", with red eyes and everything - very cool stuff!

Now, Barbieforged is an experienced player, and a bit of a min-max'er - not a problem, but I'm a narrative DM so I'm naturally a bit cautious. We get into talks about making a cool subclass or finding a homebrew one, because he feels as though the Paths that's already there doesn't fit too well with his character. Here's where the issue begins: He presents some ideas and options, and I look at them and say "well, what's the theme here? What is it supposed to look like?". He can't really answer that. I ask what he wants his character to "do" - like, what the new Path will look like and differentiate itself with. He says, he wants to be efficient and hit things with his axe. 

At this point, I'm thinking he would want a different class entirely; like a Battle Master or Duelist Fighter. I know he wants to be kind of like an Arms Warrior from World of Warcraft, but he doesn't like that idea; he wants to stick to being a Barbarian. This is where we're at now. I asked him to think about what "fantasy" he had in mind for his character, so I'm awaiting a response.

-

My problem is, basically, that I don't like making new homebrew stuff without a clear vision for what it's supposed to be; especially not when he's a very experienced, mechanically strong character who the other players often joke feels like he's a level or two higher than them. I want to give him really cool stuff; I want to make all my players feel included and "cared for", if that makes sense.

I'm also looking at it from a group point-of-view. One idea he had was a Path that let him change how he functioned in combat at certain points, but I felt like that began to infringe on the other characters. If he's suddenly super-fast and quick, does our Monk then feel like the quick one anymore? What about our 18 AC Cleric, will he feel tanky if the Barbarian has a tank-mode? Also also, I just don't like to mix "class identity" too much. Efficiency and precision is like the opposite of Barbarian to me; Barbarians are all force. Mixing in efficiency into Barbarian may make him feel cool, but at that point I'm basically crafting a class identity for him alone; and frankly, I'm not a fan of that.

Add that he was allowed to be a Warforged in the first place (the one from Wayfinder's Guide to Eberron that can change profiles) and I feel as though I'm stuck between negating his genuine desire to be a cool character and have fun, and keeping the game and myself somewhat sane and fair. Since he is the only real mechanics-focused player, he could very well just take off and become the only real powerhouse, while the other players fiddle their thumbs during combat encounters.

So, eh. Help?

I just got flash backs of Star Wars: Knights of the Old Republic and MK47...

https://www.youtube.com/watch?v=Vg1gTas7OAA

On a slightly more helpful slant...

Barbarians and Monks are both quick classes so there shouldn't be too much problem there, if he is mechanics based then might be worth seeing if he wants to multiclass to come up with some cool combo's; Barbarian Berserker + Ranger Gloomstalker fr a more sneaky/executioner style, Barbarian: Totem Warrior + Druid for some werid shapeshifting sheennigans, Barbarian (any) + Warlock the warforged is a prison for a fiend that slowly manages to exert some spells/powers through the warforged.

If he wants to remain single classed it might be worth trying to find a copy of the Path of the Juggernaught from the Tal Dorai setting (pg102) and see if that floats his boat, basic idea on its powers are you can force strength saving throw when you attack to push people around, you can't be knocked prone, your attacks gain the siege property (x2 damage vs buildings/objects/structures), bonus actions attack and its capstone is a bunch of condition immunities.

One alternative might be to substitute the battlemaster archetype for the barbarian primal path archetype and fluff the maneuvers as rediscovered combat subroutines for the warforged.

The other battlemaster archetype features would also fit with rediscovered knowledge (Student of War, Know your Enemy). They would not have any of the other fighter features. It don't think it would be out of line power wise since it is just a feature on top of the base class. Any real imbalance would depend on interactions with the barbarian base class. 

Since you are already fluffing rage as "kill mode" it should fit with the character concept and I don't think it would be too unbalanced. It has the advantage of drawing on existing features in the game rather than trying to balance something completely new. I can't see the player really objecting to that sort of approach unless their objective is to get some really OP home brewed feature.

However, I would not go that route if there is already a battlemaster fighter in the party since it would certainly step on toes.

If you find it too powerful, you could reduce the number of maneuver available by one but my first thought is that it should be ok. I don't think it would be too weak.

Thanks for your responses! We talked this morning and, somewhat out of left-field, we agreed on Path of the Zealot instead! It utilizes some things from his background (which is a little unknown to him at the moment - think Yasha from CR) and his great-axe, which has some mysterious connection to a great well of pure, necrotic energy... Interesting.

It works for both of us because it is 1) Official and well-tested content and 2) Mechanically quite powerful but focused on what it's meant to do. We both like it, and it will allow him to slot into the team's "I hit hard and won't f**king die" role, which no one else holds right now.

Thanks for your input, it helped a lot - But, do any of you have experience with similar situations? I don't want to be a hardass, I want to indulge my players' creativity, but I also can't help fret about the balance of the team and whether or not the game will be better off without their additions. Thoughts?

Give him “upgraded parts” that add cool features from other Barbarian subclasses? When you activate one of the new parts, it temporarily disables the old corresponding ability depending on what parts he has active. You can only switch parts once per long rest, perhaps even limiting that to 1 or 2 part switches per long rest.

as far as home brew abilities....I don’t really know. Stuff that is related to machinery and tinkery stuff though would see appropriate.

edit: I think what he’s actually looking for isn’t a Barbarian subclass but an entirely new machine-based class altogether.

"I don't want to be a hardass, I want to indulge my players' creativity, but I also can't help fret about the balance of the team and whether or not the game will be better off without their additions. Thoughts?"

When I run into this sort of thing my first question is "Does it fit into the game world?". The DM creates the world and each has its own character and flavor. Many of the things a player might wish for will fit into the game world without any issues. However, some can come up that are either problems for the storyline, problems for world design or perhaps just out of character for the world itself.

The next question is "Do I like the idea?". If I don't like something in my game, it might make the game less fun for me to run and in some cases for the other players to play. The person suggesting the idea may like it but others including the DM might not. If it is a minor thing related to character or background and not mechanics, I still might allow it unless whatever it might be is going to continually be a bother. Though the DM has to keep in mind that not liking another's character concept isn't something to penalize the player for. The DM has to decide if the concept could work in the game world and then more or less go with it.

"Will the idea have a negative impact on other characters or players or their experience of the game?" This could be any of the pillars, allowing a character irresistible charm might sound cool but will likely not work if everyone else wants to participate in social situations.

"Will the idea be fun for everyone?" Some folks come up with things that only benefit their characters. To be fair then, the DM has to introduce similar things for each character which can result in unintended power or scope creep in a game so if there is something that seems likely to wind up in a spiral of one-upmanship ... I'll likely choose to stop it before it gets started.

In general though, if your players have cool background ideas, want to reflavor things, want to describe or role play their character in conformance with some idea they have then I tend to encourage rather than discourage that sort of enthusiasm and interest.