Ive said this before, but realize that in any capacity, everyone is getting trashed eventually. Its all about portrayal before that moment.
Kizaru is highly likely the next one down. Then some Elbaf villain if not the Yonko or the FA. Then Blackbeard, Shanks/Mihawk, and/or Akainu (in no particular order), then Gorosei/Imu.
The reason I dont expect the Gorosei to be a true opponent here is because the last few chapters have delegated them to be these hellish monsters that support the World's King, who is the only character alive that seems to know everything.
This is why I expect Kizaru to push Luffy into mastering FS, considering he's the only one with it in the crew thus far.