Imu is too obvious. The WG is already too heavily involved and declining, while Blackbeard keeps powering up in the background.
Also in terms of sheer portrayal:
- An anomaly capable of consuming multiple DFs, even the centuries old Gorosei said this is unheard of
- Of the D. clan.
- Having Imu being concerned of and holding his poster along with Luffy
- Both his crew and Saturn himself pointing out that Blackbeard is special
- Shanks being concerned
People are fooled because Blackbeard isn't the usual fake-invincible foe who gets exposed overtime, Blackbeard is the opposite and reminiscent of Luffy himself, growing through trail and error, being vulnerable, careless, getting his ass kicked in round 1 and then bouncing back and wrecking his opponent.
And it doesn't matter Blackbeard wasn't the guy Roger was waiting for, because as Vegapunk said, the One Piece may as well fall into the wrong hands. And Blackbeard not being the guy doesn't mean he won't be the final obstacle.
So FV will either be Blackbeard or a yet unknown character. But the bigger bad is a recurring theme in Shounen. As for Blackbeard, while he was introduced very early, we still know little to nothing about his motives, his past, his character and being.