The more I think about it the more I'm convinced that martials need to just embrace 3.5 fighter design and give you a ton of bonus feats. That edition, you'd end up with 18 (19 if human) feats by level 20, of which 11 were specifically from the fighter bonus feat list. We could tone that down a bit and make it, say, 12 total (5 bonus) and let the individual classes keep some proper features to differentiate them - like Sneak Attack and evasion for rogues, Indomitable and Action Surge for fighters, Rage and Reckless Attack for barbarians, etc.
Go back to Next playtest design for maneuvers, specifically around packet 5, where they were a lot stronger and divided into general, fighter, monk, and rogue. Add barbarian.
Now, make Martial Adept better and repeatable. Let's say it grants 2 dice (at d8) and 3 maneuvers. Expand the list of maneuvers. Add level requirements so there's a sense of progression just as there is with spellcasting.
The end result is that those of us who want a more tactical, complex martial character can take Martial Adept several times along with feats like Sentinel and Mage Slayer or whatever, while those who are happy just swinging a sword every round can take simple feats like Savage Attacker and Toughness.
Also, there should probably be at least one simple caster option. Warlock is probably the best fit as it's halfway there already. Just make it more similar to, again, the 3.5 iteration, where they had only a fraction as many spells as a sorcerer or wizard, but they were all usable at will, albeit from a much smaller list and generally available several levels later. Given that short rest recharges seem to be going away, this seems like a reasonable mechanic.