What If... The Hulk Had the Brain of Bruce Banner?

What If? #2 certainly isn't when the "everyone dies" trend started, but in a way, it's still a "destroy the Marvel Universe" story. It's also distinctive because the turning point is a creative one rather than a plot point. There's no plot-related reason why the Hulk retains Banner's mind, it just happens.

What If? Vol.1 #2 (Apr 1976)
Based on: Hulk #1
The true history: After being irradiated in a gamma bomb test while trying to save Rick Jones, Bruce Banner is turned into the brutish Hulk. And the rest, as they say, is history.
Turning point: What if Stan Lee hadn't been inspired by Dr. Jekyll and Mr. Hyde? (Or in light of later events, what if Bruce Banner had never suffered child abuse?)
Story type: Deviated origin.
Watcher's mood: Kirbyesque.
Altered history: This time, and for no apparent reason, Banner's mind isn't changed along with his body when he becomes the Hulk which occasions a number of changes. First, he befriends the Gargoyle instead of fighting him, and the ugly freak now knows he's not alone (I'm sure the Berlin Wall falls earlier in this parallel). Betty Ross falls in love with Banner much faster without the melodrama of Banner's dark secret, and General Thunderbolt Ross accepts him. For the longest time, the Hulk only manifests at night (as it originally was) and never in times of stress, so Banner marries Betty and they're very happy despite the nightly size-changing (cough, cough).
He isn't an easy to manipulate monster, so Loki never sends him against Thor and the Avengers are never formed. Because super-powered egg heads are drawn to one another, Banner visits Reed Richards and together they find a cure for the Thing. Ben Grimm can then marry Alicia and the the Fantastic Four disband. But Banner stays at the Baxter Building to work on egg head stuff. They're joined by Professor X, who would rather work with these guys than create a school for mutants. When Galactus comes knocking, the three of them jump into their new Psychotron machine, which creates a super-powerful amalgam of them called the X-Man, who manages to defeat the World Devourer in a contest of wills.
When they split back into three, they've all lost their powers (which suits them fine!). However, the energies released by the X-Man turn a visiting Ben Grimm into a Hulk-level, brutish version of the Thing... and so the cosmic scales are balanced. (Sort of.)
Books canceled as a result: Fantastic Four, Incredible Hulk, and Avengers (which immediately puts Mark Millar, Jeff Loeb and Bendis out of work, moua-ha-ha). No She-Hulk either, which is really sad. No X-Men, which I'm a little ambivalent about. Of course, the Thing gets a Hulk-like title, so fans of Ben Grimm, rejoice!
These things happen: The Hulk's been through so many permutations, that this premise was bound to be used. And it was. In 1983, Banner traded in his ripped purple pants for purple speedos and even got a presidential pardon, but ultimately the Savage Hulk personality reasserted itself (I remember his fuse being pretty short already in Secret Wars, so Banner Brain didn't last 2 years). Towards the end of Peter David's run (1990), Banner integrated the Green (Savage) and Gray (Mr. Hyde) personalities into his own, and once again, we had a smart Hulk. It lasted longer, this time, right through to 1994, but events would again send Banner over the edge and into the Savage end of the pool. I'm sure a writer in the future will also want to give Banner more power over the Hulk persona, but it seems that the initial premise was a stronger one, as they keep returning to it again and again. That's the Hulk we know, and it's the Hulk we love.