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.