Fix all the wizard spells (I hate the cantrips in their current implementation)
Add all the spells available from the PHB, SCAG, PotA at least...Xanathar's would be sweet too. (why are key spells just not there....Spiritual Weapon for example).
More races especially: Dragonborn, Gnomes, Half-Orcs
All the classes and sub-classes from the PHB, some select choices from other books, including sub-class options.
Better companion options/fixes to the beast master and also to the familiars (if they're going to level up with you, give them some form of benefit)