One of the biggest hurdles that Marvel Studios has to clear is introducing the X-Men and Fantastic Four franchise into the Marvel Cinematic Universe occupied by The Avengers and co. Because Fox owned the right to X-Men and Fantastic Four until Disney acquired them in 2019, the F4 and X-Men characters were left out of the storyline for The Infinity Saga; Phase 4 of the MCU has opened up an entire Marvel Multiverse of possibilities - and you would think What If...? would be the first series to be able to give fans at least alternate versions of the X-Men or Fantastic Four.
However, according to the makers of Marvel's What If...?, using the Fantastic Four and/or the X-Men was totally off the table when making the series. During an interview with Collider, What If...? Executive Producer Brad Winderbaum explained why the show couldn't serve as a debut (of sorts) for the MCU Fantastic Four and/or X-Men:
"We had very few parameters when we were ideating the show, but that was certainly one of them. We wanted to make sure that the characters were firmly rooted in the MCU. And that had nothing to do with rights and had nothing to do with the future stories we wanted to tell. It had everything to do with what 'What If..?' is: which is a spin on an established storyline. So we look - primarily - at the Infinity Saga."
Unlike the usual Marvel media-trained answers, Winderbaum actually makes a fair point about the premise and structure of What If...?. The series is supposed to be a set of the alternate universe takes on familiar events from the Marvel Cinematic Universe; one of the key hooks of each story being that fans are already well-versed in how the original MCU movie and TV series events played out.
Using the Marvel Multiverse anthology concept to debut characters as crucial as Fantastic Four or the X-Men (or mutants in general) would be unorthodox in terms of Marvel Studios character launches, confusing for fans (would voice actors or the live-action stars provide the voice work?), and a derailing of the central premise of What If...?. More than enough reason to avoid it altogether.
Right now, the more popular Marvel fan theories predict that the actual debut of the Fantastic Four and/or X-Men will still be a ways off. The Phase 4 Multiverse Saga is predicted to culminate in a Marvel Secret Wars project that Disney may already be quietly making space for, behind-the-scenes. That MCU Secret Wars event will supposedly see heroes of the multiverse face villains like Kang The Conqueror and his variant army, and ultimately the resolution of the battle will see a reboot of the Marvel Cinematic Universe, in which new characters like X-Men and Fantastic Four will already be part of the re-established canon. If that's indeed the case, then no way What If...? would be allowed to jump the gun on those characters before their epic official debut.
Marvel's What If...? streams new episodes Wednesdays on Disney+.