Sadly I think Imu is the final villain of the story. BB will take Law’s poneglyphs, fight Luffy at Laugh Tale and die there as Luffy goes on to fight the WG/Imu
When Oda reorganized his final arc he likely decided that BB will be relegated as a secondary antagonist
Whatever Oda does with Imu it sounds bullshit kaguya bs. We don't know anything about him, his appearance, his abilities, his role, just a shadow and him "nuking" an island (said it like that as with this arc there's some serious possibilities everyone got simply teleported by some device, so it could be either way).
Him having two sets of abilities? Ok so like Blackbeard.
Him having some next level haki? Ok so almost like BB or Shanks
Him being another hidden special bs race?
Him being an ancient weapon? We go back to the point would be better to have known that a lot earlier.
Gorosei as his personal guards doing pretty much nothing for 1000 chapters + till they get faced on directly? Meh.
As you said that's very likely, but I find them lame. Blackbeard for example was special since jaya arc. He's unusual from every other character and is the closest to an actual pirate for starters. Is the only one in the verse that broke the power system rule, and again this all happened pre timeskip, well established.
Imu and the Gorosei - till the end of story.