I know it is a popular sentiment that people should migrate to Mastodon, but I personally think people will migrate wherever the people they follow migrate to.
With this in mind, I suspect Bluesky will be the platform to usurp Twitter; but with Twitter chasing off all their users in the near future, it may go to whichever platform is readily open at the time.
I'm very curious to see if Bluesky or Spill open up when Twitter closes the door on its users. If not, Mastodon may have a shot at being the next large platform...