Just guessing but the amount of space available for the engines is probably the key limiting factor. The engines on the existing A380s will be replaced multipl

Just guessing but the amount of space available for the engines is probably the key limiting factor. The engines on the existing A380s will be replaced multiple times during their lifetimes and certainly those replacement engines will have some efficiency improvements. But I doubt it's possible to change the efficiency significantly without changing the diameter and or length of the engines. And that seems pretty unlikely given how many things you'd have to change and how much it would cost to re-certify, etc.