The dead have risen out of their graves to hound Arthur Morgan in a Red Dead Redemption 2 mod inspired by Undead Nightmare. Rockstar has shied away from singleplayer DLC and expansions with its last couple of games, so this might be the closest we get to the real thing.
The video, uploaded by jedijosh920, shows off 12 minutes of zombie murder, though technically the zombies are shrieking members of Red Dead Redemption 2's horrible Night Folk. Conveniently, they look a fair bit like zombies already, though they're smart enough to carry knives.
The dimly lit evening setting certainly helps with the atmosphere, but the zombies also act how you'd expect, aside from the stabbing. Everywhere Arthur runs there are hordes of shambling corpses or poor NPCs getting murdered, and it's surprisingly effective, given that it currently relies on in-game assets.
The mod isn't finished or available yet, but you can join the Red Dead Redemption 2 modding Discord to keep an eye on it, or you can try to find collaborators for your own project.