Just more base classes, races, feats and spells - faithfully implemented according to the rules, and I would be happy.
Multiclassing would be a big boon, but it's not really that important. There's only a couple of multiclassings I would consider (Barbarian + Fighter, Paladin + Sorcerer. That's about it).
If I were reaching for a long shot, then I'd love to have the Tortle with a bunch of tortle-specific dialogue
P.S. OH! And make dialogue relevant spells like Friends and Thaumaturgy and Detect Thoughts castable IN DIALOGUE, and not something I have to precast.