Well, as said, even Gamebryo is far from the best-suited engine for open-world design. If you feel like
crying , just look at some
cryengine youtube clips. And there are plenty of them indeed.
But if it is allowed to dream about these things, I really don't see the reason why Id engine wouldn't be used, not in a same way technically as present fallout engine, but instead as an engine who combine map of the world (as in F1/F2) and open enough sections of the world, in which gameplay has been executed.
In other words, exactly same mechanic as in F1/F2, but this time as FPP (let's not talk about POV, it's not my point in this post, it could be done either way). That way they would eliminate quite lot objections to Beth's design (emptiness or overpopulated world, inv. walls, unnatural boundaries, cell problems, etc.), and easily incorporate previous in-game features (special encounters, better usage of survival, outdoor and scout skills, quest could be more... real, number of NPCs) and much easier DLC addition. Not to mention much easier manipulation in terms of dev design. As for RPG part and its use in primarily FPS engine, some modules must be written, but as long as engine supports Lua or Python (and almost all do for AI reasons), that is big but doable thing.
Technically still it would be sand-box game, though not open-world as Bethesda doing it, and I'm pretty sure that this kind of sand-box game (until someone design nice procedural algorithms for interesting world generation) is more free then, at first look, Bethesda's open-worldness with immersion-breaking boundaries and other stuff.
Of course, as Bethesda's Fallout 3 is now established "real" fallout game, we have irony too, because this type of game, most probably, would be called (if ever to happen) spin-off fallout game. But, I would be fine with that
