Let's suppose Imu is the main villain of Elbaf arc, and is getting defeated in this arc(which is unlikely), it would be a massive huge Akainu and Blackbeard upscale....
Pls Oda, don't give me hope.
I think you are thinking of typical arc contraints. But just look to Impel Down or the Paramount war. Or even WCI if Luffy never fought Katakuri.
Imu can be an overwhelming threat here without having to lose in any way that matters. If Oda wanted set up the FV in such a way that gives that character enough onscreen presense before it happens then this is a good thing.
Its the same way Akainu and Blackbeard already have had major presense in the pre-TS era. They can just be featured and lose in their respective arc/arcs.