Truth be told, even though I'm looking forward to what Oda has in store with Im, I'm not super pleased about the development from the standpoint of him being some sort of overpowered endgame antagonist — mostly because One Piece is supposed to be more of a slow-burning adventure and world-building manga with deep roots than a generic shōnen with insane power-escalation and one climactic omnipotent entity who's been inserted into the story. Especially considering he's being introduced now — after 2 whole decades, and isn't part of the One Piece fabric. Even in say Naruto, I'd have preferred a long-standing character who's woven into the lore than Kaguya as the Big-Bad (Griffiths from Berserk is more along the lines of the former, and marvelously executed). A lot of interesting theories floating around regarding him, though...
e.g.
https://www.reddit.com/r/OnePiece/comments/8rc05o/chapter_908_theories_with_reference_to_japanese/
As for Blackbeard, I'd much rather he squared off vs. Hiken no Sabo. While I love the Strawhats and them getting extended panel-time, they shouldn't be fighting almost every single antagonistic Yonko — as well as the World Government. Plus, he never really struck me as a suitable final villain because while he's devious and has close to no moral code, and is in some ways the polar opposite of Luffy — who's still very naive, he really lacks the commanding aura of the likes of Roger and Whitebeard at their peak, or even Kaido — plus, it's hard to take him seriously when a looot of his power is externally derived from the Yami Yami no Mi and Gura Gura no Mi.
Wrt. the Final War, I really like the idea of folks rallying around Luffy against the World Government and whoever opposes him as the Pirate King — seems like a really organic course of acton. Though looking at things right now in terms of power-scaling...I can't quite see how the World Government is going to contain the Pirate King, his crew (including the two monsters) and Grand Fleet, the Red Hair Pirates — who'll definitely support the successor of Roger, Dragon and Sabo, etc. unless they collude with Blackbeard (maybe feed him more fruits), and monopolize the Ancient Weapons. Maybe that's why Oda has introduced Im — to tip the balance in favor of the World Government and create extra tension.