Other storyline NPC, such as Jondar, Mira Eamon, Moek, Larel and Karel brothers, or Zoltun Kull's body parts guardians will respawn as well.
It will be a huuuge + for D3 if travel to another acts will exists, and all bosses will be at their places.
I know, there's something which doesn't fit to this idea, the Nephalem Valor system.
Bosses like Skeleton King, or Ghom can drop new Archon recipes. But first we have to collect 5NV.
With open world it will be too easy. Just collect 5NV once and kill all bosses.
But i think there is a way. After travel to another act by waypoints we loose not all NVs but maybe 2 stacks. And after kill boss who gives us extra reward, we loose some stacks too.
Nephalem Valor system isn't bad, but it is a fact, we loose some freedom because of it.