That doesn't have anything to do with the engine. Think of it is this way, you can move around in the game, so the camera can move around too. If the camera can move around it can also be moved to where your head would be for 1st persion or it can move somewhere else to make it 3th person.
Indeed, I'm sure that with the Gamebryo engine, a turn-based, isometric game could be made just as easily, but since the Elder Scrolls series has always been first person, I see no reason to worry about this changing. Really, the main concern I have about the new engine is how moddable it would be. It is safe to assume that the new engine will bring many technical improvements to graphics, sound and physics, that is as inevitable is night passing into day, but we have no assurances on how well it will handle mods. I also hope the new engine will be more stable and efficient than the Gamebryo engine has been, but I'm not getting my hopes too high, Bethesda has never been known for making well optimized, stable games.