Blackbeard is 100% the final villain of the story and easily one of the most compelling characters in the series, if not the most compelling.
Blackbeard has some kind of connection to Rocks and I believe he has known about Imu since he was a kid.
I think Blackbeard searched practically his whole life for the Yami Yami specifically to beat Imu
and[/I] steal his power.
Given the Holyland is burning, I think most of that will be revealed THIS arc.
I think Elbaph is much longer than people think and it ends with Imu's death or "death" as he's cast down by Blacbeard succeeding where Rocks failed.
All that being said - I think Imu's more mysterious because his identity is tied so much we don't know:
- Joyboy & Ancient Kingdom
- D Clan & Will of D
- Twenty Monarchs
- Ancient Weapons
- Devil Fruits
- End of the Second World
- Void Century
Imu will reveal much and more in this confrontation.
If he has lived for centuries as the only one to ever use his DF, assuming he has one, Blackbeard may involuntarily merge with him in some sense.