I don't think that Luffy has to always be the underdog for the fight to be exciting. We are now going to follow the strongest character in the verse. I think that is pretty cool. He is going to fight guys with trickier powers, like Blackbeard, so he is still going to have a challenge. Also that gives the story a chance to focus on the rest of the One Piece universe.
Think Oda will give Teach a big win somehow before the Luffy vs Teach, to make Teach bounty bigger than Luffy, and most likely bigger than WB one also. To keep the underdog theme for the fight for the One Piece.
After that its just Imu, that is different from the other villains, because of his connections with JoyBoy