Yea FATE's will be really good in my opinion. The combat part would be something like the RIFT right? Never played that MMO but i believe its the same philosophy.
The real problem for me with systems like that, where you just cooperate with a bunch of strangers, is...
It's really impossible to make them difficult or engaging. I mean, you're just tossing a sh*tload of people at stuff, and it ALWAYS turns out to be a mindless zerg.
So yeah, I'm very startled and scared at the prospect of FATE-only open world content.