Ya know, the younger me wants it to be the Wolf Empire, just because that's what I grew up on as it came out new. While the older, wiser, and more tactical me wants it to be the Star Adders...
I think Catalyst will throw in something from left field that has been literally right in our face the whole time. Hinted at in various books like a puzzle. Something just so strange we wouldn't have seen it without disecting every book & TRO out there.
Stone being a vat born Wolverine. Yes I know there was a fight between him & Alaric and he lost, and the thought of this is just stupid yet insane at the same time for SO many reasons.
Or something like the Fidelis say screw it, we fulfilled our promise and are taking the seat on Terra and reclaim their Smoke Jaguar identity (this could actually work considering they were supposed to be annihilated by Task Force Serpent, so they weren't abjured by the Home Worlds, therefore could be a legitimate ilClan).
Or the Wolverines or Blood Spirits themselves come out of nowhere (considering they are both supposed to be long dead) and sneak in the back door.
Just something crazy ya know?