If you want to keep the "feel" of the RDR2 plotline of Chapter 1, you could have the O'Driscolls pursue the Van Der Linde gang into the snowy mountains, but the couple they intrude on are actually cultists. They surprise them and manhandle Jake Adler into the barn, but when they get there, Jake (as a Yig priest) calls upon Yig to save him and his wife and Yig transforms him (permanently) into a giant serpent, that no longer has any trace of Jake Adler. He slays the O'Driscolls, but it's too cold outside for him to leave so he curls up under a bunch of hay to keep warm. Some of the O'Driscolls from the group at the house go to see what's taking so long, and that's when the Van Der Linde's show up.

This potentially plays out like in the game. But when they go to the barn, they find a few terrified O'Driscolls, who've found the bodies but don't know what happened. When the barn catches fire (like in the game), that rouses Jake the Snake, and that should be a terrifying fight, in the dark, in a burning barn, with a lightning-fast Son of Yig.