Now I'm imaging a version where he becomes an ultra wealthy faith healer on the backs of his predictions. He wins a couple of times only to be arrested for fraud but continues to make predictions from jail and claims God is whispering to him these winnings so he can give them away to loyal followers.

He gets out of jail only to be met with a large following which he uses to build a cult of personality around himself as the Vox Dei. The betting houses won't accept his predictions anymore so within his cult he rewards the most loyal with a prediction under the pretext that god is rewarding them for loyalty. He doesn't have to take anything from these people, as they just donate most of the winnings back to him.

He builds a religious empire and eventually forms his own religious offshoot of Christianity and turns the town into a kind of mini Utah. Just like the original script, he has Marty's dad killed, but uses his position as a prophet to forecast his dads death, as a warning to Marty's Mom and an example of his prophet like powers. The subplot is that he convinces Marty's Mom that he has reformed and found God and thus is good and righteous, which through movie plot nonsense she believes.

The town could have all kinds of alt reality nonsense. Like instead of signs for the mayor race they're replaced with wanted posters for revolutionary figure Goldie Wilson. The court house is transformed into a megachurch. Marty's school is now a religious school and his principal is a zealot for Biff. Everyone thinks Biff is a great guy and can't understand why Marty hates him so much. Everything about the town and its people is centered around Biff, and unlike the original script where it becomes clear fast that something is extremely wrong, most things look normal. Until Marty finally finds his mother, working at the megachurch as its director. It's revealed that the current Marty in this time is a missionary and everyone is confused as to why he's back.

Biff is actually super generous and kind right up until he figures out that this isn't the real Marty, but instead the Marty he was warned about by the old man who gave him the sports almanac. At which point its revealed that under this image of god is still the same greedy, cruel, Biff. Then you get to have that scene with the two of them in his office. It's revealed that Biff keeps the sports almanac hidden inside a fake Bible in a safe in this office, that he killed Marty's dad, he used his death as a pretext to turn Marty's mom towards religion and eventually fall in love with Biff, and now he's going to kill Marty.

During the chase between Biff and Marty, Biff is relentlessly hounded by his followers and employees, he has to maintain his holy peaceful image in front of them, but eventually snaps, pulling his gun and shooting towards Marty as they race through the megachurch towards the roof. It can play out mostly the same, and Docs subplot could be mostly unchanged as well.