Leaders, probably. Because the random factor of when they appear is so large, and also because milking weak enemies for leaders is pretty artificial.
(Perhaps there should be some other way to build the Forbidden Palace, if there's no leader, though. Or perhaps it would just force more compact kingdoms, instead of the dumbbell shape.)
I'd also like surplus food and shields to carry over, instead of being lost, to reduce micromanagement, but I think that's more like adding a feature than removing one.
I'd also like changes to how culture flips work, although they don't need to be eliminated. Distance from the capital should add or subtract, not just divide the probability, so that if a city is very close to my capital and far from my opponent, it will never flip away from me, or has a good chance to flip to me.