I'm going to leave out UI improvements because I assume they're given.
I'd REALLY like the option for a 5 player party.
I would love to see all of the PHB and Xanathar's classes and subclasses implemented. Heck, I'd be perfectly fine with the PHB options now and the Xanthar's ones added in a $60 expansion later. Spells and feats from Xanathar's, too.
Speaking of Xanathar's, the Tool Proficiencies chapter is nice and elegant and it'd be cool to see that implemented. The game seems full of tools that don't do anything right now, anyway, so, maybe?
I'd also like to see more weapons with more differentiation. (See thread on longbow vs short bow range.) This could be just more fun magic weapons, but maybe also use the optional "speed factor" rules in the DMG.