I wouldn't be surprised actually if they intentionally left out the sub-classes for whom they themselves actually would prefer to make "new stuff" for. That was one of the original issues we say in the 3E era... all kinds of things WotC might've wanted to publish books about further down the line had already been done many times over by other third-party publishers.
Considering the very slow, heavily-outsourced, publication strategy we see going forward that seems tantamount to blocking 'new stuff' entirely in the near term.
But as it stands now... only they get to make new 'Battlemaster Maneuvers' (mentioned as such). Only they get to make new 'Totem Warrior' animal styles. Only they get to expand on the 'Wild Surge' table. Only they get to make new animals which can be specifically mentioned as being compatible to the 'Circle of the Moon' druid for wildshaping, and the 'Beastmaster' ranger for companions.
They're also the only ones who can riff off those things or reference them, which could be a tad chilling when it comes to developing closely-related things. Especially given 5e's design paradigm, in which mechanics are more tightly coupled to the fluff they represent. Each publisher who wants to put out a few new maneuvers or a class or sub-class that uses maneuvers, instead having to create a novel maneuver system, rather than just tack onto the battlemaster's or throw a class a few CS dice. That would make the new content from each such publisher incompatible or at least, inconsistent, with eachother, and the base game. Conversely, adding a new school of wizardry or a new Domain, or a few new spells could be done fairly smoothly.
Asking why something should be created strikes me as carrying the insidious implication that creation requires justification, and that without said justification, something doesn't deserve to exist.
There's been a lot of that going around since the edition war.
I do not think it would be too hard to come up with a martial maneuvers system that uses variable damage dice for weapon that scale (just like spells scale with level). That would include weapon properties and things like stances to totally rewrite the martial side of things.
It wouldn't be. It might not be that hard to come up with a better one than the Battlemaster has (JMHO, but it'd be hard not to do better, MDDs from the playtest, for instance). But, such a system would be untethered from the official game, while one that built on and improved the battlemaster system would have some level of compatibility and sense of continuity. We might very well get several very good alternate maneuver systems, none of which get any visibility, while a not-as-good extrapolation of battlemaster maneuvers might have a shot at broad acceptance.
Overall it's inconsistent and complicated, but not yet bloated, much like the classic D&D it captures the feel of so well, it's just familiar enough complexity that it's not a barrier to our enjoyment.
