The problem with turning martial classes into spellcasters is that you’ve solved the problem by erasing martial classes. A lot of the other proposed solutions have similar problems.
I'm going to say that The Hulk is a perfectly fine high level fighter and is no spellcaster. Allowing martial characters to do what the heroes of e.g. The Song of Roland, Outlaws at the Water Margin, Celtic Myth, or modern anime, action movies, and superhero comics can do
in no way makes them spellcasters. The Hulk, Beowulf, Hercules, and CuChulain should be all examples of high level fighters (whether Batman's a fighter or rogue is an interesting question).
And if anyone complains that Hercules is a demigod then we can take the demigod and angelic inspiration (e.g. Gandalf) out of wizards
Or we can level cap everyone.
To begin with, let’s look at one thing that D&D has IMO done right when modelling martial heroes – hit points.
On the contrary hit points are what
absolutely cripples martial heroes. A solid mid level martial hero should be able to one-shot an ogre, reliably killing an unarmoured ogre in a single attack. The mere fact that a 5e fighter or barbarian might as well be trying to beat it to death with a nerf bat.
Hit points, or rather hit points for everyone, are most of what
prevents martial characters being as lethal as they should be. Yes the martial should
have hit points. (And healing surges). But hit points should protect far more against magic than they do against cold steel.
My experience with 1e and especially 1e post Unearthed Arcana is that fighters just ruled large portions of play with such massive advantages in hit points, easier access to high AC, saving throws, and THAC0 that they were juggernauts. #
I have precisely no problem with this. Wizards ruled
other large parts of play. So did clerics. When the class name is "fighter" and the class itself is about fighting with no out of combat support it should rule the only pillar it uses.
Should "fighter" be a class? Possibly not. But if that's the class then it should be the best at the only thing it does.