Honestly just hope whatever tech they used hasn't eaten up too much of their time.
Bethesda RPG's are some of the most intricate games ever created, if not patently THE MOST intricate games; the amount of missions you can do, the order at which you can do them, the way they can affect each other.. the way killing/stealing/etc. can affect what you can do, etc. Then you have the sheer amount of objects/bodies/etc. that are left around the world, most of them indefinitely.
I would be completely fine with them using their old engine with some new effects / higher resolution assets baked in.
Skyrim w/ mods for instance still looks phenomenal when you take into account the scope of the game. Happening upon fights between trolls and dragons.. shit is nuts!