I've always believed Blackbeard is the main villain, but given recent revelations, I'm wary of Blackbeard taking down Imu, only to then be possessed by Davy D. Jones. I'm hoping that isn't true.
I'm not too worried about Imu's characterisation. I believe Oda will develop Imu's character while he's still alive
and after Blackbeard usurps him.
The biggest Lore Piece revelations happen through flashbacks and I fully expect to have at least 4 major flashback arcs left
- the definitive Roger flashback
- the definitive Blackbeard flashback
- the definitive Shanks flashback
- the definitive Void Century flashback
Whatever order it happens, we will learn more about Imu in each of them.
That being said, I think we are overestimating Imu's role in the story. He is a relic of the Second World. The prophesied Great War for the fate of Third World (and possibly the birth of a Fourth), will come down to Luffy and Blackbeard.
I don't think Imu is long for this world the moment he leaves Elbaph. Blackbeard is lurking.