Cap'n Kobold
Hero
5th ed tends to use a "Recharges on a roll of x+ on a d6" for NPC mechanics that aren't just a set number of times per encounter.The crusader had got a random system to recover spent maneuvers, and this was too complicated to be used like nPCs by the DM, but maybe like a solo boss.
In 5e, Ki is explicitly a kind of magic.My opinion is the first step to reintroduce the martial maneuvers could be new subclasses, and later something like the pathfinder archetypes when some class features are replaced by others..(gladiator class from Dark Sun would be an example of D&D version of Pthf archetypes).
We need to differentiate between maneuvers with ki power source, or magic. Ki should be a power source like arcana, divina, primal magic, or psionic.
Splitting magic into arcane, divine, primal, psionic etc is very much a 4th ed thing, not 5th ed. If you're going to do that, why not put ki back in psionic?
You already have Samurai (Paladins of the Crown, Champion Fighters), Ninja (Shadow Monks, Rogues of almost any persuasion), Monks etc.Martial adept classes can't be only fighters with martial maneuvers, but they also need their own mark of identity. That is the reason when I try to create a martial adept as character my mind thinks about samurai, monks, ninjas, sohei and other oriental classes.
Tying a system into a distinction based on real-life race and culture doesn't strike me as something 5th ed goes for. Even the original Bo9S didn't, outside of Desert Wind tending to be from cultures with deserts.
For classes, both superiority dice and Ki points refresh on a short rest. That would seem close.* The key is the martial maneuvers are neither at-will nor once-encounter powers, but a middle step between. PCs need a special action to reload maneuvers, like the psion who spends his focus and he want to regain it with a concentration check. The reload of martial maneuvers has to be a fast and easy system to can use martial adepts as nPC enemies (or fights will be slower).
For NPCs, the recharge mechanic would seem to fit well if you want something between once-per-encounter (considering the NPC is only likely to exist for one encounter) and at-will.