Not to necessarily replace 5e as it is now, but more for optional rules (in no particular order):
1) racial ability penalties
2) magic item shops/wealth per level guidelines
3) DR against magic types (silver, adamantine, holy damage, +3 swords, etc.)
4) minions
5) ability score requisites for some classes
6) streamlining saves (when do you ever have a Cha save?)
7) healing surges (spreading out non-magical healing to free up the cleric)
8) combat maneuvers with mechanical benefits (disarm, trip, bull rush, etc.)
9) lethality (not sure what rules would need to be added/changed to reflect this)
10) better encounter building (instead of the weird multipliers we have now)