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Cap and IM seemingly died at the end of Time Runs Out. Ultimate End doesn't make much sense generally.

Deadpool is out next year, if that does well they might want to go for something similar.

Otherwise something magical if Strange does well, or maybe an Avengers spin off? Vision/Scarlet Witch?

I'm guessing it's nothing to do with rights and more that either it sounded dorky or that "quantum realm" sounded cooler.

The majority of the alternate Dooms and Reeds were wiped out in Hickman's FF run.