For my money…
Assassin as a Class
Swordmage and Warlord classes, though I’m not attached to either name nor specific mechanics from 4e. 
Bard songs a la 3.5, but as bonus actions. 
Level-gated feats to do the job of paragon paths and prestige classes. 
Multiclass feats and hybrid multiclassing. 
Playable Gnolls!
Class feats, maybe…
Like a lot of class stuff is abbreviated by 5e’s design. Totem Barbarian could lean harder into being transformed by invoking a spirit into themself, to become the wrath of nature embodied. 
Rewrite thrown weapons to not suck. 
Magic thrown items should just return to hand, as in 4e.