This is my first theory and I'm making this at 3am so stay with me
During the post-credit scene of Thunderbolts* we see the Excelsior coming to earth
But obviously it's not the actual Excelsior since this one has 4 legs while the actual one has 3. So who is it?
During the F4:FS trailer we see Reed writing down equations and calculations for what I'm heavily and loosely guessing are for universe hopping in case Galactus destroys their planet and or universe
At some point in time while the F4 is fighting and or preparing for a fight to the death, someone is able to get the equations and builds his own ship to save his own skin and go to another universe to cause havoc. That man is Tony Stark—or Dr Doom.
RDJ has been confirmed to be playing Dr Doom which seems like a very weird choice considering how recognizable his face and voice is, but in the comics Tony comes back. But since our Tony is dead, they just grab another one from another universe—specifically the F4 universe.
Tony could easily build a replica of the Excelsior with his own touches, but make it look similar to the actual Excelsior to leave his earth without being as noticed, since civilians would think it was the F4 going to stop Galactus
Thank you for coming to my sleep deprived TED talk
