>>5975
With you opening the door to different universes you have a lot of tools at your disposal. Let's see

Fred/Forest Fire goes off on a quest to stop the 3rd party.
This quest needs to have him seek out aid and have to do something for somepony else other than himself.

If universe hoping is involved, how about another Equestria as the other faction? You could have plenty of more exotic choices; like a universe that was ruled by the same Tyrant Celestia but was overthrown and they are starting to meddle is this CB bureau's affairs? A crazed pony from a ruined world who wants to destroy all alicorns regardless? A Princess Celestia from a world that is trying to subvert both the CB and Earth? Possibilities are endless.

Though if you are looking for a player from your CB I still think there are several candidates that could be a compelling antagonist. You could have it be some figure from within Equestria who is out to kill all competition. How about a power hungry Blueblood with an Alicorn Amulet who has set up shop secretly somewhere? If you have Cadence exist you could have her as someone who is in change of CB brainwashing with a more horrifying version of her vaguely established emotion powers. A trip to a warped Crystal Empire that was on the edge of the bubble when Equestria first teleported over and bore the brunt of a brief Human retaliation could be a pretty cool setting if you ask me.

>The deus ex machina is clearly a frindship-magic blocking ring. It would let him cast spells up through mid-unicorn energy levels but also make him functionally invisible even to earth ponies.
>The castle is already rife with multiverse holes and in most other worlds that metal acts as anti-magic
hmmmmmmmmmmmmmmmmmmmmmmmmmmmmm... Where does he get this ring? If this ring blocks magic how is able to cast spells (or is friendship magic just the magic of the earth ponies.)