I don't know if you'd count magical items as "player material"... but I would definitely like to see more magic items in the book.
I would, and I agree!
Also, more feats, Fighting Styles, Ki options for Monks, a revision of the ki cost of Elemental Disciplines if possible, one or two new pact boons for warlock...
New subclasses for most classes that aren't spellcasters, that gives options similar to the monk's ki abilities, of the battlemaster's superiority dice/maneuvers. So every class can have at least one subclass where the player gets to pick from a list of options at more levels, and has interesting distinct abilities available during play.
More variant race features.
More races.
Rules for Organizations, similar in purpose, and hopefully in general style, to Star Wars Saga Edtion's Organization rules.
Optional Skill Points rules. preferably not 3.x style, with the multiclassing weirdness.
A way to build a warlock style half caster. So I can cannibalize it to make my ranger cast spells warlock style.
IDK, whatever else is in popular demand.