@ the bolded; Yeah, this only bolsters the Imu-sama final villain camp, as it's fully expected Luffy will "shoulder the centuries of history" and confront the World Government.
@ the rest;
Whitebeard outright says Blackbeard's "not him", then goes on to describe an individual who will inherit Ace's will and shoulder centuries of history which directly leads to the great war... Then mentions the world being turned upside down with the discovery of the One Piece.
I'm not sure how a world where the governing body is dismantled (via Teach) can be turned upside down even more. And that's ignoring the fact that the One Piece has obvious historic ties to the Void Century and will be integral in the instigation of the Great War.
Teach has no place in this. He cannot take down the World Government because Whitebeard outright says he's not the one to do so and Blackbeard cannot be the world government because he doesn't represent the world government, he'd be an usurper at best.
Cool, the race for the One Piece can be even more dramatic. That does not give him any role beyond it, though.
I'm not sure I agree with your assessment here.
Blackbeard could have plans to take on the World Government and cause horrible, unimaginable pain to the world, after becoming pirate king. Luffy fighting him there would keep those stakes high because if he loses, Blackbeard can go on a rampage.
There's nothing that requires Blackbeard to be the final villain and per Oda's own build up, Teach cannot be the one to take down the world government.
Nor does he currently pose a fraction of the threat that they do.