I don't see the thematic split as two, but four in One Piece.
- Good Marines/WG (order, saving people) = Represented by Sengoku, Tsuru, Garp
- Bad Marines/WG (power and kill anyone who disagrees) = CDs/Akainu
- Good Pirates (Freedom seeking sailors) = Luffy/Shanks
- Bad Pirates (Freedom that descends into anarchy) = Blackbeard/Davy Jones
Since this is a pirate manga, I don't think the ultimate thematic battle will be control vs. freedom as presented by Imu, but about different kinds of freedom.
Do you want the individualistic freedom to explore, wander, etc. like Luffy, or the freedom where there is no morality, no accountability, and everything descends into chaos. People can murder with no consequences. Just the strongest, or cunningest, survive. That's what Blackbeard represents.
I think Imu's backstory became clear. He wanted to stop the "bad freedom" brought on by the Davy family. The complete chaos that leads to anarchy and destruction. But I think ultimately he will fail, and it will be the "good freedom" that defeats, not control. That's why I think Luffy vs. BB will be the end.