Destructive only to an extent though. I don't want to see holes blown in the side of walls (pretty sure game mechanics of inside/outside cells would prevent this anyway). However, taking down small trees with a fireball would be cool.
Farcry 2 did this very well with fire on the environment.
Hm, that makes me think of something else. Relevant to your post, but irrelevant to this thread.
Anyway, what I'm thinking of is to not to seperate inside/exterior cells. I mean, that seperation is basically (from the start) to cut down on the FPS hits. But most games, if not all by now, have no seperation from this. Maybe this could have its own thread though.
And it's kind of late to discuss, I mean... if Bethesda has implemented it, it would have been implemented from the start, and not now. So it's not like anything would change.