About players and mobs: As far as the battle engine is concerned, they will both be derived from a base class, so any functionality they can share, they will. I haven't considered putting mobs in your party, but you have to consider the implications of such things, such as gaining experience, leveling, wearing equipment, etc.
PvP isn't planned at all. I did once consider co-op -- consider, for example, the Phoenix Cave from FF6 where you split your party (or Kefka's lair). I'm not sure how well that would play out, though, and it would certainly have a major impact on development, so it's not going to be a focus for a while.
ATB will be wait-only in v1, but active will be implemented. And for hardcoding vs. adding lots of options, it's definitely going to be an iterative development process. At work, we've been very successful developing small, releaseable units, and I definitely want to keep that methodology.
When we get something testable, I'll definitely open it up to get some talented pixel pushers creating sprites and tilesets. I think one of the trickiest things to do will be making a completely original quest with a coherent vision that people will actually want to play, as an anchor for the modding community to build on.