OK, so how is the Multiverse of Madness going to work?
From what we can gather so far, with various heroes and villains from non-MCU film incarnations appearing in upcoming movies it is clear that the Multiverse already exists.
First I should note that the destruction of the Infinity Stones did not create the Multiverse; instead, it unraveled the Marvel MCU Universe – also known as Earth-199999 in the litany of Marvel Earths for the comic-book Multiverse.
And now, apparently, because the Universe of Earth-199999 is unraveling, it is crashing into other Alternate Universes – and other Marvel Universes in the already extant (but unknown to residents of Earth-199999) Multiverse are crashing into it.
There exists a storyline in Marvel Comics that fits this exact scenario – the World Incursions that led to the Time Runs Out crossover event that led to the second massive Secret Wars event.
Now, there is a LOT in that series of stories that just simply won’t work in the MCU. Lots of the characters involved in the MCU are already dead; there is no “Illuminati” in the MCU as most of the characters in that organization don’t even exist in the MCU; and a lot of the super-Cosmic stuff that went on just won’t work for the style they have created for the MCU.
But a lot of the basics from that series of events can be adapted to Phase Four and Phase Five of the MCU.
From the Marvel Fandom Wiki:
“The events known as incursions are the result of a contraction in the Multiverse's timeline. They exist in a paradoxical nature, as this contraction was caused by the early death of an unknown universe approximately 25 years in the past, when its Molecule Man was killed by the Molecule Man of Earth-616.”
Of course, Molecule Man does not exist in the MCU; in the case of the MCU, the precipitating event is the “destruction” of the Infinity Stones in Endgame.
In the Time Runs Out series, when two Alternate Universe collide, they are after a short time destroyed. Now, that is not going to happen in the MCU… at first. But that will be the end result unless the process of “Universal Contraction” (the “Multiverse of Madness”) is stopped.
Phase Four is going to show us the beginning of the process, and in the end, the discovery that, without something to stop the process, the entire Multiverse will collapse in on itself, bringing an end to everything.
Well, everything except the re-forming Infinity Stones.
So at about the same time that it is discovered that the Multiverse will collapse in on itself, it will be discovered that the Infinity Stones are reforming, and that the key to stopping the collapse is gathering the Infinity Stones to “set the MCU Universe right” and thus stop the process – Phase Five.
But until then, the process of collapse continues, until like in Secret Wars, all but a handful of Universes are destroyed, and perhaps then we see the formation of BattleWorld.
Possible, but we will see. It seems a little “super-Cosmic” for the MCU. Maybe, like Sakaar in Thor: Ragnarok, we will see a “BattleWorld,” but it isn’t a bunch of Alternate Earths (including Earth-199999) glommed together, it is something else… shards of other Earths glommed together, likely leaving Earth-199999 intact but still endangered. But you never know…
Note that it is interesting that Marvel has started bringing back World Incursions only just this summer, in the X-Men title Excalibur; see Excalibur Just Brought Incursions Back Into the Marvel Multiverse.
So… would this not be an excellent way to bring not only the Fantastic Four, but also the recently-acquired X-Men into the MCU?