The sequel makers are going to want evil force using, red lightsaber weilding bad guys ASAP. So I suspect we're going to see yet more secret Sith masters who were hiding out reveal themselves. Not some new overwhelming force from beyond the galaxy which would make more sense.
Sounds very prequel-ish, but I imagine that's how these movies will go. The Empire is defeated. A slow Phantom Menace mystery may be all they can turn to.
We'll see.