Basically what's in the meme. I don't know if this is an appropriate post here, but I don't have all the specifics.
The gist though is pretty much as described - limit break is its own resource(Like a channel divinity charge) that you can use for Action Surge and then, perhaps, a new ability for each subclass. These charges would refresh on short rest(that's important) and you could maybe even gain a third ability of your choice at higher levels, and of course you gain more charges as you level.
Champion's could be called Honour-Bind and act as a sort of pseudo Compelled Duel ability, to force aggro on you(advantage on Opportunity Attacks, Crossbow Expert negated if it isn't against you, bonus to AC for not having any allies within melee range but having multiple enemies within melee range, etc).
Banneret could get Rallying Cry as this instead of just a pathetic add-on to Second Wind - The user and everyone who hears it gains 5 or so turns of a bunch of Temp HP, immunity to frightened, advantage against poisoned and charmed, some sort of bonus to damage dealt with attacks, and then maybe even instantly stabilizes if they're on death saves, or even just straight-up gets up at 5 HP.
This is made with the idea of BattleMaster maneuvers being resourceless and available to all martials in mind(hence the title), but if it's still a subclass it could just outright gain a huge bonus to its maneuvers for a time.
Rune Knight could just slightly buff its third level Giant's Might ability(The thing that makes you beeg) and have that as its Limit Break.
This was also made with the idea of making Barbarian a fighter subclass in mind, and of course, Barbaric Rage is that subclass limit break.
Maybe even Ranger, and a version of Hunter's Mark becomes their Limit Break? For instance, you could split into three different versions - Pinpoint, which would give you bonuses to attacks against your mark(which are different, but not necessarily strictly better or worse, depending if they're melee, ranged, or even with spells), Chase, which would speed you up and make it easier to slow down, grapple, tackle etc your mark(or pick it up or steal it if it's an object), and Stalk, which...well, lets you hide and stalk your mark, maybe even allowing bonus action dodging. Maybe that's a bit too many subclasses for one class, though...
This was obviously conceptualized for D&D, though I think it would work in other games like Pathfinder just as well. Any sort of tips for what exactly to do with this?
The martial/caster disparity is the (IMO, proven and obvious) idea that martial characters lack gameplay options compared to their caster counterparts, and that this problem only ever gets worse with level.
Also, "martial" in this case specifically refers to Fighters, Barbarians, and Rogues.
IMO, it does exist, but it's not as "end of the world"-bad as some people make it out to be. Basically, rogues are fine because they get a crap-ton of skills that can be put to good use as long as the rogue player makes their character with even a little long-term thought. Rogues that have problems tend to focus in things like stealth, and other physical skills that casters can use spells to imitate or replace. Rogues that pick up and spend expertise in one or two soft skills (some kind of knowledge skill, insight, investigation, etc...) will never find themselves with nothing to do and will always have a niche where they can make the full casters go "holy shit!" from time to time.
Fighters and Barbarians actually have problems because they seem to have been made more with dungeoncrawling in mind, to the detrement of anything non-dungeon related. They generally lack useful soft skills, and don't stack stats that will make using them useful because they generally don't have ways to make a high int or wisdom terribly useful.
Fighters and Barbarians compound the skill problem by not gaining useful/impactful abilities in T3 or T4. When full casters are busy choosing and enjoying the most powerful spells in the game, fighters get another use of indomitable (which never, ever fucking works, IME), a second action surge per short rest WAY too late for it to really matter, and a 4th attack they will probably never, ever actually get because it's at 20th level for some stupid, fucking reason (as opposed to level 17 where ALL casters, even the half-casters, get their 4th cantrip damage die).
Barbarians get even less than fighters due to most of their class budget being tied up in a massive passive ability: Brutal Critical. So all they ever get to do is crit-fish, which they've all been doing since level 1 anyway.
The disparity is choice and impact. Because of their lack of choices, it can seem difficult to have an impact on the game, mechanically. A good DM can make up for this in a variety of ways, but when you're just looking at the rules or white-rooming a character, the problem does tend to become a bit obvious...if overblown.
Generally, the fix is simply to give fighters and barbarians more class abilities that involve getting to make interesting choices during play.
Also, “martial” in this case specifically refers to Fighters, Barbarians, and Rogues.
Don't discount Monks! They're generally regarded as the worst class in the game, as they're essentially fighters that deal less damage in exchange for a resource that lets them use cool abilities...for a short time. Yeah, I know they're technically? magical, but they function and present as a martial class, poorly at that.
The disparity is choice and impact. Because of their lack of choices, it can seem difficult to have an impact on the game, mechanically. A good DM can make up for this in a variety of ways, but when you’re just looking at the rules or white-rooming a character, the problem does tend to become a bit obvious…if overblown.
Also don't discount, being able to play out-of-combat scenarios. Casters get utility like Pass Without Trace, Spider Climb, Prestidigitation, Friends, and then the fact that most of them are charisma-based for some strange reason, which is THE out-of-combat stat. I know my suggestion in particular doesn't really address that, and is more focused on the more-abilities and/or allowing interesting choices part that you mention after that, but I do absolutely want things like 3.5e's Moment of Perfect Mind, and whatnot.
That's how bad monks are. I forgot they even existed :D
But seriously, they've been getting some good changes in the UAs, and with WotC being a bit more generous with things like expertise it makes being useful a lot easier when you've got a lot of random weird shit you can do, like a monk can.
Honestly, Monks are so close to being a top tier class. All they really need is for WotC to pull their heads out of their collective asses and make short rests not a fucking tooth-pulling exercise in frustration for your average group.
If it were me I'd bring back the concept of 10-minute "exploration turns". You use one exploration turn to do something like pick a lock, break down a door, climb your speed x 5 in relatively safe conditions without a check, attempt to disarm a trap, attempt to climb a slippery or dangerous surface, examine a magic item (arcana check to figure out something basic), make a general knowledge check about a subject (would have to define a distinct difference between using knowledge checks in combat and using them outside of combat), etc...
...but, most importantly, you would use an exploration turn to try and take a short rest. Take one short rest action and you can spend one hit dice per three levels (rounding up). Take a second in a row and you can spend one hit dice per three levels. Take three of them back-to-back and you can spend another one hit dice per three levels, and any short rest recharges trigger. Your short rest is now done. You can gain the recharge benefit of the third rest action twice per long rest.
...and while that was going on the rest of the party was able to fuck around and do stuff. Which means the monk who gassed themselves in the last combat can take three rest actions to get back their ki while the rogue searches a room, disarms a trap guarding a hidden chest, and then picks the lock on the chest.
I fundamentally don’t understand why classes having a different amount of options is a problem. But then again, maybe I just have brain rot from decades of playing B/X and BECMI.
In the older editions, like the ones you're talking about, casters had serious downsides. Between being very fragile, spells being interrupteable, and sometimes having different XP amounts, casters were kinda 'glass cannons', and needed a martial frontline.
In 3.5 and 5e, casters have had these harsh downsides decreased or removed, while not otherwise losing power. They are more or less strictly better than martials, in the sense they can do 90%+ of what martials can do better than they can do it, while also doing several other things. And the few things martials do do better, it's by slight degrees.
It's not just that casters are powerful, it's that they're powerful and flexible, able to be top tier in several different roles at the same time, and can change what roles they cover by resting and swapping spells.
Whereas martials can sometimes build to be top tier in one role, but they're largely locked into that one role, or can build to be okay in several roles (and be outclassed by casters in all of them).
You put that WAY better than I did.
And yeah, At this point Casters really can just do everything martials can, better, and more. There's just no denying that.
Yeah. It was worse in 3.5 ironically; despite casters having more downsides than 5e, spells were overall stronger. It did leave this narrow window at levels 1 and 2 where martials were basically strictly better, but caster quickly skyrocketted in power, especially if you were playing with prestige classes.
Spell power was reigned in for 5e, and pretty sharply at that (most notably from adding Concentration). But, they also washed away caster downsides, by making cantrips at will, casters not quite so fragile, and by softening Vancian casting. 5e is still absolutely more balanced than 3.5, but that's not saying a lot; 3.5's power level was all over the place.
Still, I feel like 5e's levels 1-5 are pretty balanced, and the martial/caster imbalance doesn't really become painful until like, level 12.
When power levels of the degree of modern TTRPGs are expected of characters, D&D sort of becomes a game all about your options and which you choose when. If certain characters have inherently MORE options to pick from in such a setting, that means they are essentially objectively better by default.