I don’t think Imu being revealed changes the endgame villain, it actually reinforces why Blackbeard makes more sense. Oda has had the core trajectory of the story planned from the beginning, and the Yonko, especially Blackbeard, have always been positioned as the ultimate obstacles in Luffy’s path to becoming Pirate King. Blackbeard isn’t just another antagonist, he’s Luffy’s direct opposite and narrative rival, built up since before Jaya as the dark reflection of the same dream. On top of that, he isn’t acting randomly, he’s clearly carrying on the will of Rocks (and pushing it further), but doing it in a far more calculated, patient, and manipulative way that consistently puts him in positions to win. There’s no real reason to suddenly doubt his trajectory when everything about his character points toward eventual success, including potentially surpassing the World Government itself and claiming the throne Rocks couldn’t. While Imu is undeniably one of the most important and dangerous figures in the story, representing control over the world and its hidden history, they function more as the embodiment of the system that needs to be broken, not necessarily the final personal obstacle. Blackbeard, on the other hand, is actively moving within that system, taking advantage of chaos, making strategic plays, and positioning himself at the center of the race for the One Piece. We already have significant reveals about Imu, the World Government, and the Void Century, yet Blackbeard and his crew remain full of unanswered questions despite being in the story much longer, which strongly suggests Oda is saving their full reveal for the climax. Even now, Blackbeard’s crew is making moves that will inevitably clash with the World Government, especially with the Revolutionary Army destabilizing Mariejois, which is exactly the kind of chaos Blackbeard thrives in and exploits better than anyone. At its core, One Piece is about the race to the top and the clash of dreams, not just the dismantling of a regime, and that places Blackbeard directly in Luffy’s path as the final and most personal obstacle.
(TLDR, Imu represents the world that needs to fall, Blackbeard is the one built to rise above it and stand as Luffy’s final enemy.)