improvement: advantage rework (backstab is stupidly OP, high ground should only be available to Obi Wan Kenobi, make flanking a thing), then action economy rework (amognst them, make jump an actionless movement without disengage ffs)
new stuff: paladin, then dragonborn
Backstab used to be way more broken, because it could be used on a per attack basis, not per round. Kingmaker keeps the 3.5e backstab.
But it does make a lot of sense to gain advantage and backstab when flanking an enemy from behind.