Warning, highly speculative. But you knew that!
It was a super gradual thing so it'd be hard to pinpoint an exact set of events, because keep in mind that even before he started getting a little too close with Darth Sidious he was already having fantasies of power and possession. But I'll just go simple and say that he wouldn't have intervened - at least significantly - in Palpatine's arrest. So Mace Windu would have gone to arrest Palpatine after Anakin alerts him that he is a Sith Lord. Palpatine would have been subsequently defeated (though likely R.I.P. to Kit Fisto, Agen Kolar, and Saesee Tinn) and killed, and Master Windu would have lived to see another day. Sadly the Clone Wars would have still happened, but Padmé would have also survived, giving birth to Luke and Leia who would have probably been inducted into the Jedi Order. As for what would have happened between the two, with their marriage and all, that's a bit tricky. One could argue that his arguably toxic relationship with her was the cause of his turn and therefore it's a bit difficult to envision a scenario in which he "stays on the Light" while still being with Padmé. So he'd have to either come off the relationship or there would have to be some reforms and communication (a more likely option given that they both still loved each other in some shape or form).
I went to r/MawInstallation to see another answer, and here's one I found quite interesting (by u/zwands):
"I think Anakin would still be likely to gun for the top spot in whatever new-world-order was going to develop. At this point, he had already begun thinking about ruling the galaxy, and becoming stronger than the Jedi. He already felt betrayed by, and too weak within his current organization.
I could see him taking his portion of loyal clones, and potentially seizing what firepower he could from the Separatists, and extending the fight with Palpatine directly.
I also see this being twisted as a new-Jedi versus Sith battle, so he’d retain Obi-Wan and other survivors, but with a lower tolerance for anti-Tarkin-like strategy."
That's almost fanfic worthy!
