Howdy there! How's it goin'? Welcome to this post. :)

I've gotten bored recently and have been trying to make some homebrew features for martial classes that would function/be similar to the way feats were/are thought of in 3.X and starfinder. I haven't played any 3rd edition games, but playing starfinder did show me how feats for those systems were much more specific and a little niche, rather than the uberpowered, "Why didn't you take war caster at 4th level? It's a caster essential." way feats are constructed in 5e. In those systems you get so many more, and they help "fine tune" the way your characetr plays in combat and a few other things. This isn't to speak of any powercreep in those systems, but the general concept is still there. I've seen some folks say that battle-master maneuvers should just be a thing all martial classes get, and so that's sort of how this idea came about as well. 

So I made a little google doc of my idea. It's called Techniques. Essentially mini-feats. Smaller boons and actions for martial characters that aim at giving them versatility, customization, and other things to do than just attack.

Here it is: [x] <- Link

A small summary for you folks: Barbarians, Fighters, Rogues, Monks, Paladins, and Rangers are the only ones that get Techniques as a part of their class. You get 1 + your Intelligence modifier at level 1 (minimum of 1) and then 1 at 5th level, and another at each odd level after that. If you mutliclass, you only get 1 Technique at the 1st level of the class you're dipping into and then follow progression as normal.

I'm posting mostly for ideas to add to this document, what technique would you like for me to add? OPness and balance aren't my main concern yet, as this is still the brainstorming stage, but do keep in mind the scope of Techniques. Something like the Athlete feat would be 3 or 4 separate techniques (not including the ASI) and each would likely be considered OP. An average fighter PC with only a 10 INT would still have 9 Techniques, so they shouldn't be as strong as feats as they are meant to have more of them.

You could have it scale of when you get ASIs, maybe instead of feats you can get two techniques, or you can gain a +1 to a stat and a technique or something, which would give fighters and rogues some more options.
I've been thinking about something like this for a while, it feels like it might be a bit much, but I'll give some ideas.
You could open your document up to suggest changes and comments and I could give some.

Crossbow Training. (Could be a fighting style, maybe you could combine fighting styles into this or remove them since they're not especially interesting)
You ignore the loading property of crossbows for the purposes of attacking multiple times. Maybe another small bonus like a +1 to attack or something cool, I'm not sure.

Duellist.
You add your damage roll to checks to disarm and can choose to deal your ability modifier in damage.

Sweep.
When you attack with a two-handed melee weapon, you can deal damage to another creature within 5 feet of your original target. If your attack roll would hit the other creature, it takes damage equal to your Strength modifier.

Elemental Battle Magic. (Maybe a level prerequisite)
Once per turn when you deal damage with a weapon attack, you can deal an additional 1d4 fire, lightning, cold or acid damage (or more types?). (Or something, just a +damage isn't super interesting, but gives a neat 'magical warrior' type of feel)

Unknown Name? Something like sentinel or... I dunno
When a creature stands up from prone within 5 feet of you, you can use your reaction to make a melee weapon attack against them.

Another Shielding type ability
Once per short rest when you are in an area of effect, you can grant yourself and each creature of your choice within 5 feet of you resistance to the damage taken, or a bonus to the saving throw?

Might have class-specific techniques too, maybe one for barbarian is something like...
When you rage, you can expend an additional rage use to gain 1d6 temporary hit points at the start of your turn until your rage ends. When you gain unlimited rages, maybe you can do it a few times per rest.

Or for fighter at a higher level...
When you use your Indomitable ability to reroll a Strength, Dexterity or Constitution saving throw, you automatically succeed. Additionally, you gain one extra use of Indomitable.

Lots of options and cool things you could do.

I originally did have it scale with ASI's and then decided against it after my friend pointed out that these are more like martial eldritch invocations. And so I had them scale in a similar way. I think it fits better for you to get more of them and for them to not be tied up with feats and ASIs.

I think class-specific techniques are a great idea! I can possibly ease-off any presumed OPness by giving high level requirements, such as to the technique for the fighter you mentioned.

So far the one about hitting creatures who get up from prone is the one I'm looking to. Trying to think of a good name.

The idea behind having it as ASIs is that right now you're just increasing their power level on top of what they already have, rather than just increasing the choices they can make. Making it cost a feat doesn't increase their power, but just adds to the interesting things a martial character can do since they're giving up GWM or +1 th and damage or something.

Just adding things on can screw with the balance of different martial, especially if it's based on a stat and some classes might not have the ability to invest in being so MAD already, like monk or barb or paladin. It's an interesting concept, though it does screw with the things D&D has set in place. I saw one idea called 'master strikes' and it was a spell slot style system martial got past 11th level to let them do cool stuff which I thought was neat. Maybe these techniques can only be taken past the 5th level or 11th level or something since low-level D&D is pretty volatile, small changes like that can swing the balance quickly.

Feats are optional. Adding them already messes up the Bounded Accuracy and power level of all PCs in the first place.

The point of Techniques though is to give more versatility and such. There are several low level spells that have more text, conditions, and complexity than most martials' class features.

If multiclassing and feats aren't turn on, how do you think techniques would do to the base game? Because the base game wasn't made with the expectation that everyone would be using multiclassing and feats. So considering the power level with them and techniques, while valuable, does miss the mark in my own opinion. I say this whilst still considering their optimization opportunities with feats anyway.

In actual combat there are many small moves, techniques, and strategies that one can utilize and specialize in. I want to try and bring that in with my little system, and so them having more techniques than just the ~5 they MIGHT get (if they focused only on techniques instead of ASIs or feats)  makes more sense for that.

My bad, by feats I meant ASIs, though I hoped you understood my point. I was trying to say that dropping this on top of martial classes without adjustments or cost could screw with it. I was trying to say that sacrificing ASIs is a way you could add a 'cost,' maybe you can say one technique is equal to a single +1 to an ability. I dunno, maybe this is a bad way to do it. 

Really this is all speculation, run a game with it just dropped on top and it could work absolutely fine, I have no idea.

I would suggest raiding 3.5e, 4e, and other games for ideas, pathfinder, zweihander, maybe even video games like guild wars and stuff like that.