Good question! Does anyone have any good homebrew or 3pp to address this issue? Because otherwise, what are you all trying to accomplish?
That would require someone putting their money where their mouth is and designing a solution.
Nearly 70 pages in and we cannot even get people to agree that the problem even exists and you are insisting on seeing the solutions?
I bowed out of this dozens of pages ago and then sauntered back in to see how things had fared since. And, oh look, the same merry-go-round of absolute refusal to accept that the problem is even a problem.
I mean, we had a sidebar about Wish. Never minding that I can simply Simulacrum a copy of myself and have that cast wish until it burns out, then wash/rinse/repeat. All for the low low cost of 1500 gp. Infinite wishes, and since one of those wishes can create money, it literally IS infinite wishes. Now, what can your fighter do that would even REMOTELY come close to that?
Heck, if you really want to go super cheesy, simply have your Simulacrum cast Simulacrum. Get about ten copies of yourself - true, they don't have many HP by then, but, they've still got your full casting ability. They are 100% under control, and add a full suite of actions to every round. Your fighter gets eight or ten or even more attacks per round once per short rest? That's cute. I've got 10 9th level spells in a single round. Beat that.
One of the problems I aim my design at in my SRD project is that casters have far too many spell slots at mid to high levels.
Wow, FINALLY someone actually admits that there is a problem. I'd LOVE to see casters get stripped down. But, in core D&D? That's never going to happen. For exactly the same reasons we see in this thread - You'll never get enough people to even agree that the problem exists in the first place.
At the end of the day, I actually kinda agree with
@Clint_L. This could, and actually probably should, be resolved through subclass. The problem is, if you go that direction, then these new subclasses would be substantially more powerful than existing ones. Which would garner such a huge backlash, again, because we cannot even get people to agree that the problem exists, that any such subclasses would never, ever see print.
Imagine it this way. A subclass - which means only this class of fighter (or whatever) gets these abilties which grants, say, special movement (limited fly/swim/climb speed, something like that) at 9th level, area of effect damage similar to a ranger's Volley ability at 11th, the ability to dispel/disrupt magic effects at, say, 13th, and then other goodies later on. There's absolutely no way you'd get that into print.
Like I've said all the way along, this conversation is futile. The decision has been made. Fighters absolutely will never come anywhere near what casters can do at higher levels. 5e D&D is Potterverse where virtually everyone is a caster except for Hagrid, who is kept along because, well, he's a fun character and who wouldn't want to play Hagrid?