The main game would certainly be a lot more replayable if people could play as the monsters. Greater power for DMs would be nice too, though I think it would be harder to pull off in Divinity than in NWN because Divinity is turn-based and NWN was sort of real-time turns. I don't think there's any way as of now to have players in separate parties, so that's functionality that Larian needs to add for that to be possible unless some genius modder can really figure something clever out. Someone at Larian said the game could theoretically support lots of players, probably at least 7 or 8, but that wasn't really tested so there could be issues there.
Honestly, everything takes so much time right now in editting that I can't even imagine someone being able to design combat scenarios and stuff on the fly. But Larian should be making the editor a bit easier to use, so maybe that will change.
