Technically, they've been using the Morrowind's engine since they released Morrowind. They just kept adding stuff to it and increasing it's capability to do more.
Kinda the difference between a Model T and the Monster Truck Death Model T. Yeah, it's still technically a Model T, but they've changed so much about it...
So expect a new and improved Gamebryo engine for TESV.
This makes my soul happy. As long as it doesn't "Feel" like oblivion. I'll be more then happy. I want new experiences. My FO3 and FONV experiences are mixed due to the fact they "Feel" the same.
P.S is there a term for the "FEELINGS" you get in games? (When you walk, die, kill, jump, attack, ect)