Oda is showing Imu too much for him to be the final villain.
I'm pretty sure this is it, the moment Imu will return to his room, BB will be there to finish him off and steal his power.
Again, no actual build up to that. No reason for Blackbeard to strike NOW (because he doesn't know Imu would be vulnerable; until BB can be sure of that i am 100% certain he won't make his move), he has a whole other sub-plot with the WG and wanting to become an official nation via leveraging hostages (Koby and now Garp] that hasn't been addressed yet, and Imu is in the deepest recesses of Pangaea Castle anyway.
Even if Blackbeard could get in there, past all the Marines and HK's and Gorosei and get a shot at Imu could be REALLY beat Imu neg diff in the Flower Room (even if Imu is weakened after all this, would he be THAT weakened? Really? That BB of all people could beat him with zero push back? The guy who gets heavy hands put on him in damn near every fight even when he has a sizable strength advantage?) and not get jumped? Highly doubt it.