What's the design space for new primarily "martial/non-caster" classes? Are there "martial/non-caster" concepts players have that current classes aren't meeting?
There has never been a fantasy character concept that
couldn't be made to fit into one of the core classes, with minimal loss of class identity.
The real culprit here is third edition, with its flexible multi-classing. Once players had the option to change class every level, a lot of players noticed that there was no reason to continue in a non-casting class after it stopped granting new powers, which is why 3.5 (and even moreso with Pathfinder) tried as hard as it could to make sure every class gave a new power at every level. If you remember the early polls for 5E, even, one of the big demands was the removal of "empty" levels.
And that meant the creation of filler content, which served to differentiate classes in trivial ways. You could remove fully
half of the abilities from every martial class, and it wouldn't change the game significantly except in reducing its complexity. But now that we've established a precedent for trivial abilities differentiating martial classes, many people seem to feel that they need new classes with different trivial abilities in order to properly represent a character concept.
For instance, Mike Mearls once worked on Iron Heroes which created a whole bunch of "martial" classes for that game. It can be done, and one of the lead D&D designers has done. The question is would it add value to the game? Is it something we as players want? Does it fill a unique design space?
I've read through Iron Heroes, and it reminds me a lot of 4E in that everyone has a lot of decisions to make every round, and every combat would need to take several rounds in order to really explore the tactical depth that they expect you to use. I'm also reminded of the Book of 9 Swords, and the way that its myriad power options
didn't always compare favorably against a PHB fighter just making a full-attack action.
Maneuvers worked well in 4E because they intended the game to be
about fighting. You did some exploring and talking between fights, sure, but you were expected to have four fights per day and those fights would each take an hour to resolve. Fighters had maneuvers, and wizards has spells, and everyone's turn took about the same amount of time to resolve.
I don't really want D&D to go down that road, though. I don't think that fifth edition
wants to be that game, either. If given the choice between juggling maneuvers with different cooldowns for an optimal dpr of 37, or auto-attacking for a dpr of 35, I'll choose the auto-attack
because I want combat to be over so we can move on to the next thing. That's the real benefit of abstracting all of those maneuvers into a single attack roll. If there's a disparity in spotlight between the time spent to resolve the fighter's attacks and the wizard's spells, I would much rather that it be addressed by reducing the complexity on the wizard's side.