Since I'm running a 13th Age game, I'll throw the question on its head; what do I miss from 3e / pathfinder / 4e that I am going to use in 13th Age?
The flippant answer is "nothing", but although 90% true, I do miss a couple of things:
- Spell Components. Stupid, annoying and pretty much always ignored as they were (unless they cost gold pieces) I liked them.
- Rituals; I think 13th Age is more ritual-friendly than 4e was, and it would be fun to see "ritual-only" magic as an option
The rest, not so much. So the list of stuff I'd use is more like "everything except the above". If it was an ordered list, I'd go for:
- No-grid combat; glad to see that go
- Losing iterative attacks; nothing guaranteed to make fights more boring than the fact that full sequence attacks are so much better than single attacks. Instead boost up the melee attacks so a single attack does the rough equivalent
- Off-turn actions; slows the game down too much
- XP gains for killing. Hated the "wait, wait, let's go back and kill the orc for it's XP" mentality
- Flexible attacks especially for melee types. Given the static nature of 3e fights, this would inject some interest for the fighter
- Escalation dice; easy to add