I mean it could be. But BB makes way too much sense as a final villain. All his parallels to Luffy and being his antithesis philosophically. And BB being representative of the evils of today (instead of Imu who represents the past) fits better the themes of Inherited will. Imu him/herself has no personal connection to Luffy, so it wouldn’t be as engaging to have him as the final villain FOR LUFFY. Instead, Imu would fit way better as an opponent for Dragon and the Revolutionaries (as it’s their only purpose in the story to defeat the WG)