To my knowledge, there have been two "What if Captain America were revived today?" what ifs. One of them has Red Skull pretending to be the real Cap and taking over the government.
The other is far more 90s and has shows a world that's basically been split down the middle with Dr. Doom ruling one side and Red Skull ruling the other.
Has this even been done -- What IF Charles Xavier founded the X-Men with Colossus, Nightcrawler, Wolverine, Sunfire, Banshee, Storm and Thunderbird?
Assumption here is that when the original teams was coming into their own -- Xavier wasn't properly motivated to from a school/team -- and so those characters may have become villains in the same manner as Quicksilver and Scarlet Witch -- or they may have become heroes on their own -- I can see Iceman and Angel doing that. The question is would Xavier have sent his new team to Krakoa -- and would have had to formed another team to rescue the first?
What if Ultron had never turned on Hank Pym and joined The Avengers?
Ultron taking on the Vision role of first 'constructed' Avenger could be interesting.
Another option could be the 'memory engram' process Pym used to awaken sentience in Ultron might have gone too far and ended up transferring Pym's brain to Ultron, making Ant-Man's next upgrade not Giant-Man or Yellowjacket, but straight to 'Pymtron.'
What If Secret Invasion had been a success for thew Skrulls?
Those could be very compelling stories, especially if it turned out Ultron was just biding his time until he got secure enough in a position of power and/or trust within the Avengers for him to be able to execute his plans.
We kind of got that in the official What If? for Secret Invasion, with the Skrulls even using some kind of genetic hybridization treatment to turn consenting humans into Skrulls, only then they all died at the end because Norman Osborn, having gotten to use the process on himself, disguised himself as Iron Man and infected them with a lethal virus tailored to Skrull DNA.
The spider is always on the hunt.
What Ifs back in the day sometimes came across as 'Take That!'s instead of What if, where the result of any change to the 'official' storyline made things exponentially catastrophically worse, and felt, once or twice, like the writers directly addressing any criticism of the original story by suggesting that if it 'hadn't happened their way' it would have been horrible. (The What If cartoon on the other hand, hasn't felt like that at all, to me, and has had some genuinely happy endings as well.)
In that vein, I would prefer an AU where Ultron was *not* a bad-guy, biding his time to exterminate humanity. He'd take on the Vision role in not just the 'constructed person' part, but also in the 'robot hero' aspect as well. One way to differentiate him from 616 Ultron might be to have him be on the team as it faces against other constructed foes, like the Kree Sentry robot that decides to kill everyone on Earth to free up it's resources and territory for the Kree empire, and lead to Ultron recognizing that not all robots or AIs are natural allies...
That said, it doesn't all have to be puppies and rainbows either. Ultron might start out with a bit of a learning curve, being made of indestructible metal, he might not fully recognize how utterly fragile most organic life is by comparison (or how hard to repair if it's brutally damaged...) leading to some excessive force at the beginning, and mistrust from other Avengers (since he'd be seen as dangerous as the Hulk, in his own way!). He, with the Pym engrams, might also have a bit of a torch for Janet, which, if Pym remains an independent person, could lead to conflict (cue the Vision / Wonder Man fight over Wanda, only it's Pym and Ultron over Janet!), or, if Pym's brain is gone, and Ultron is all the remains, some Vision/Wanda 'inter-species relationship' drama if 'Pymtron' and Janet actually do continue their relationship...
Lots of directions this could go, and I'd find one in which this What If Ultron *wasn't* a genocidal AI supremacist more interesting than a paint-by-numbers retelling of a more traditional 616 Ultron story. (Then again, I'd be tempted to keep some of the oogy Oedipal stuff, no matter how questionable it got from a psychological standpoint, because I likes me some messy drama...)
"We live in a world of cowards. We live in a world full of small minds who are afraid. We are ruled by those who refuse to risk anything of their own. Who guard their over bloated paucities of power with money. With false reasoning. With measured hesitance. With prideful, recalcitrant inaction. With hateful invective. With weapons. F@#K these selfish fools and their prevailing world order." Tony Stark
What if Spider-Man stopped the burglar so that Uncle Ben lived? We've seen a couple where Aunt May died instead, but I don't recall one where both Ben and May lived. Without that early tragedy to drive him, I could see him continue being a celebrity instead of taking heroics seriously.