The MCU isn't holding back anymore. It's at the point where any story it churns out can help build up its universe even further and can also be pretty much well-received. Suffice to say, the whole cinematic universe is only getting started with What If...? which is an anthology of alternate universe stories that imagine what could have been if something changed in the sequence of a hero or the Avengers' origin stories, among many other prospects.
So far, it's been a wild ride and it just keeps expanding the possibilities in the MCU and its multiverse at a breakneck pace. All of that will culminate into the next phases of the Avengers mythos. It just so happens that there's a ton of What If? stories just waiting to be explored in the Marvel comic books and some of them would translate well into the ongoing series.
10 Deadpool Kills The Marvel Universe
Marvel and MCU fans have been clamoring for him ever since Ryan Reynolds was cast as the perfect Deadpool. He's yet to make an entrance in the MCU but Disney could easily wedge him into the multiverse with a crazy idea from the comic books: what if Deadpool fought the Avengers?
Well, scratch that; this notion is just too big to be just one episode. In the 2011 comic book titled Deadpool Kills the Marvel Universe, Deadpool goes more haywire than he already is and decides to kill everyone in the Marvel Universe, including the Avengers. It's a comedy horror that would work better in animation than live-action due to how outlandish it is even for a comic book story.
9 Iron Man Was Evil
Some Marvel comic book stories were so controversial that they got erased from the canon timeline after they caused more trouble than they're worth. This one notably happened during The Crossing story arc. In it, Tony Stark was Kang the Conqueror's sleeper agent all along— something he hid for a long time.
Stark even had to off several people just to maintain the secret and carry out Kang's plans. Ultimately, the Avengers prevailed and had to import a younger Tony Stark from another timeline only for the "real" Tony Stark to return. With Kang's introduction in Disney+'s Loki, this might well be prime picking for an episode.
8 Thanos Joins The Avengers
Speaking of controversial, here's a big one. Thanos, one of the most prevalent and powerful Avenger nemeses, gets a role reversal in What If Thanos Joined the Avengers? It doesn't get less imaginative than that. In it, Captain America wasn't present anymore so Thanos took up his role and led the Avengers with a cost.
He's still Thanos, however, but he decided to help Earth's mightiest heroes in assailing other worlds. That is, with the help of his own version of the Avengers. So it's not the same Thanos as the one whom T'Challa tamed in MCU What If...?'s second episode.
7 Aunt May Becomes Spider-May
The idea of the radioactive spider choosing to bite someone else has already been explored well in the Spider-Verse. However, that film forgot to include one of the most outrageous Spider-Person candidates available: an aging Aunt May.
This was explored in What If? #23. Aunt May from Earth-3123 was, of course, bitten by the radioactive spider. Soon after, she became Spider-May who has more creative uses for webs than web-slinging, such as knitting a sticky sweater for an adolescent Peter Parker.
6 Peter Parker Becomes The Punisher
Aunt May isn't the only one to get her What-If? treatment. Peter also has his fair share of parallel universe fiction but perhaps the most interesting of them all would be when he turned into the Punisher. This took place relatively recently, during What If? The Punisher back in 2018.
It should have been the typical Spider-Man origin story except Peter Parker decided to kill Uncle Ben's murderer. After that, his course in the timeline changed drastically where he modified his web-shooters to also fire lethal projectiles. More than that, his costume also took a darker turn, literally, as it also sported the iconic Punisher skull.
5 The Punisher Becomes Captain America
As for the Punisher, he also gets his place in the What If? fiction. It's not clear whether the MCU wants him in after a successful Netflix outing but he could get a warm welcome as Captain America's replacement in What If...? Such was the possibility conjured up by What If? #51.
That one takes place in Earth-93070 where Steve Rogers lost and perished against the Red Skull. Nick Fury had to find a replacement so the military readily recommended Frank Castle because of his combat proficiency. Castle joins immediately after losing his family but his stint as Captain America was problematic thanks to his murderous tendency.
4 The Hulk Goes Berserk
What If? #45 suggested something that the Avengers have always feared: the Hulk actually going out of control. As such, the comic book issue is aptly named What If the Hulk Went Berserk?!. It's one of the rare What If? titles that have a serious exclamation mark.
It doesn't really take much for the Hulk to become a mindless killing machine. Bruce Banner here merely failed to save Rick Jones from the gamma explosion that created the Hulk. This prompted major changes in the Hulk's behavior, resulting in the Avengers' heavy hitters responding in panic.
3 Thor Gets Raised By Frost Giants
2018's What If? #1 has something special for Thor fans: Thor getting the reverse Loki treatment and getting raised by Frost Giants. King Laufey somehow gets his hands on a young Thor and decides to raise the Odinson kid into his own. The Frost Giant leader then fuels Thor's impulse and destructiveness.
This time around though, it's Loki trying to play the good brother role as he's with his original Frost Giant family. There's also a Frost Giant equivalent of Mjolnir named Ice Crusher which also became an object of Thor and Loki's resentment towards each other.
2 Loki Changes Mjolnir
This alternate timeline, namely What If #47 (1977), assumes it was Loki who first capitalized on Mjolnir's power and thus wielded the magical hammer. It's low-hanging fruit, obviously as the orphaned Odinson has always wanted Thor's power for himself.
The big difference here is that Thor from 1977 is a different version of the character. He has a human alter ego named Donald Blake who only serves as Thor's host. Thus, Loki makes Blake's life hell as he tampers with alternate universes and tries to manipulate Mjolnir's allegiance.
1 Avengers Versus JLA
Apparently, this huge studio crossover event is canon according to Marvel but some strong fan opinions beg to differ. In any case, it's still technically a What If? story since the ending somewhat reset everything to a point where the whole event almost didn't happen. The issue is aptly called JLA/Avengers.
Regardless, it's all thanks to a certain villain who can manipulate reality that these two ensembles of caped tights and leotards get to meet and battle their equivalents. In it, the heroes finally get to settle some age-old debates about who's stronger, faster, or smarter. Should the MCU go that route, it would help their DCEU competition get a leg up.