Feats: War caster, Sentinel, Pole-arm Master Spells: Counter spell, Shield, Vicious Mockery. Classes: some of the Xanathar's stuff. College of Swards. Cavalier. Just bards in general. I know they're coming but I love bards. I hope they can come up with a good implementation for bardic inspiration.
Removal of disengage, hide, and shove from bonus actions. I like the idea of adding more baseline bonus actions, but this is really stealing the thunder from a lot of classes and feats. Rogues cunning action, pushing strike, shove as a bonus action from shield master.
Grappling game play. A bit niche, but there are some fun grappler builds out there.
Expertise. One of the best parts of being a rogue or bard that is not in the game yet.
A better system for reactions. I think it is legit a bit trickier to implement in a game, but reactions feel bad right now coming from 5e. You don't have much flexibility and you have to anticipate rather than react.
Dodge action, ready action, and help.
Better party pathing so my group doesn't run through a bunch of traps and into some goblins because I jumped over a pit.