The top things I want to see in 5E:
Psionic classes. They are a core mechanic from prior editions that many of us have incorporated into our setting that we've had to work around not having official rules to handle.
More feats. The existing options are fine, but some of them need to be upgraded to be worthwhile, even though they fill a good conceptual home. There are also a lot of ideas for feats that have been proposed that are not explored.
More spells. There are a lot of historic spells that do not exist, and a lot of 5E specific concepts - like ways to combine concentration spells in a balanced fashion, that have yet to be explored. I have about 1000 homebrew spells in 5E as I like each spellcaster to feel unique, but many of them are described on a few words (as 3E, 10d6 dam, dex sav 1/2).
More magic items. I'd like to see a book that has 500 or so magic items - there are a lot of historical items from prior editions that have not been included in the current edition. As with spells, I have a lot of home brew items, but
And as a bonus - get out of the books. I want to see WotC offer DM University, a set of classes that DMs can take online to improve their skills. It should cover administration, acting, world building, handling common player problems, adventure type recommendations (dungeon delve, versus mystery, versus heist, versus war storyline, versus hex crawl, versus.... - how to prepare for an adventure of each type) WotC could work with D&D Beyond to do this and then create some unique boards on the Beyond site for DMs that have completed the courses to discuss ideas outside the prying eyes of (most) players.