I honestly don't know which route Oda will go, but I personally think Blackbeard as real final villain over Imu would be a bad choice.
Sure, Blackbeard is the most stereotype pirate in the series and therefore primed to be Luffy's main pirate rival (it's a series about pirates at its core after all). Same with his crew of ugly pirate basterds.
However, the moment Oda decided to introduce a tyrannical world government with a primordial "God"/"Devil" ruling as king of the world (who basically created this whole world and now tries to sink it), the deal was sealed.
The latter is just so much more important in the overall context of the world, you can't sell me BB as final villain anymore.
Even if Oda decided to go the Blackbeard route, it just won't feel like the final villain.
He is completely insignificant in the lore of the One Piece world (only relevant as a pirate for 2 years, whereas Imu has been the literal dictator of the world for 800+ years) and also doesn't mean anything to the vast majority of citizens in the OP world (99.9% haven't been affected by Blackbeard at all, whereas everyone basically lives in "Imu's dystopia world" under the reign of the CDs).