Regarding side-factions: If you wanted to say that they are actually are a multiversal agency, you could flesh out the TSAB as a UMMA-friendly side faction.

As for original factions, hmm... maybe some sort of religious group that believes that each world in the multiverse has a fragment of some sort of ur-scripture hidden in its own legends. Their goal would be to spread their own faith as wide as possible, so that they could find more fragments. They could be friendly or a bit to overzealous, depending on how you want to swing it.