Although I do have my Views™ about
Cats vs. Dogs Marvel vs. DC Casters vs. Martials, one thing I would like to see is more formal support for multiple styles of play in the core rules.
For instance, IMO folks who feel the current situation is satisfactory (or at least satisfactory
enough) and folks who want a higher ceiling for martials, and folks who want a lower ceiling for casters should all be able to sit down and play D&D out of the box and feel they're getting what they want out of the game, even if they might feel unsatisfied playing at the same table as one another. IMO the likes of
@Oofta and
@HammerMan should both be able to have fun playing D&D in more or less the way they want, even if they might not have fun playing together at the same table.
I state that because D&D, on account of its market share and history, does present itself to some extent as a game for everyone. Likewise, the promise of such modularity was discussed at some length during the playtest period. Should not a game with pretensions to universality have the (mechanical) guts to back up such a pretense?
With that stated:
(1) One of the selling points of D&D 5e in particular is its streamlined, rulings-over-rules nature. This nature is also a strong part of its design aesthetic. I don't think it's to the good for the game writ large to go the way of 3.X, much less systems such as GURPS or RIFTS. So it would be for the best for added subsystems to try to stick to that aesthetic. In fairness, this might mean that at least some (most?) "gonzo martial" stuff might have to be relegated to a supplement - although I daresay that if so, such a supplement should be very swiftly published after core rules. (Maybe some of the existing "gonzo magic" stuff should be so relegated as well?)
(2) Here I should note that there's still plenty of room for supplements to support less broad playstyles, the way Van
Helsing's Richten's Guide (oops!) goes into more detail about running a horror game, or the way Ghosts of Saltmarsh adds new layers to waterborne adventure, so they're not cluttering up the core rules while still expanding the way people can play the game, for those as want such things.