Blackbeard is the final boss of the series and I think oda has made that pretty clear imo.
1. BBs first target was Luffy and would have dogged his ass at the time
2. Instead, Blackbeard captures Ace and sets up his execution. Luffy holds him personally responsible.
3. Introduced as an exact foil with a mirrored dream, more of a rival than any other Worst Gen pirate.
4. Consistently growing in power and stays several arcs ahead of Luffy
5. Blackbeards darkness fruit is the thematic opposite of Luffy
6. Oh yeah, did I mention he got Ace killed for a Warlord title he immediately used to shit on Luffy's new friend who carried marineford
Akainu was just doing his job, so while Luffy hates him and might even want to kill him, Blackbeard only did it for the clout. And so Akainu humbled Luffy, and Blackbeard infuriated him.
Imu sounds like the natural choice, but this is a manga about pirates and Oda claims that things wrap up shortly after finding the One Piece. A pirate fight for the One Piece fits better, and depending on how Blackbeard folds there might be no one to mourn. Hence Oda gets to have one big blow out party funded by the treasure found.
edit: and if the WG could locate laughtale or follow one of the pirate crews there, they'd just Mother Flame all of them at the same time.