Which is a fascinating thought because if One Piece is needed to topple the govt to achieve said goal, then it would imply that Blackbeard (in this case, hes not the protagonist) "would" have to lose prior by that logic.
IMO, Rafter has some "trial" system to prevent the wrong people from getting the OP, even when it is "time to claim it." I can see the SHs being exhausted by the "trial" and the immense lore dump one such island would require, which is where the BBPs swoop in, steal the OP, and trap the SHs on Rafter. By the time the SHs make it out, the world has already been turned upside down, with PK BB in possession of both OP and at least one ancient weapon.
I think IMU will be dealt with before sailing for Rafter, and that Shanks will guide the SHs there after a pre-"trial" held by the RHPs to test the SHs. To forshadow BB's switcheroo, while we get the SHs finally reaching the OP and learning what it is, we might get a little cliffhanger of the BBPs also reaching Rafter and fronting on the RHPs, with one of Shanks' iconic "You won't stop the future blah blah". Right before BB steals the OP, we cut back and see a bloodied Shanks dying on Rafter's shores.