Are there any games outside of the Half-Life series that you could plausibly do a HLVRAI or Half-Life Full Regret thing with?
Like it kind of needs to be sufficiently well-known that lots of people know what the baseline is supposed to look like (so that you can then diverge, either in the plotline or just by doing silly things in co-op that don't make sense in singleplayer), the engine needs to be moddable (or at least have transferrable data) so that it's relatively straightforward to make a co-op version of the original singleplayer game, there should probably be a rotating cast of nameless generic ally NPCs or whatever so that other players can slot themselves in with their own characters, and the story has to be all in-engine with no cutscenes or text boxes so you don't have any interruptions and so that the "generic allies" played by other people can talk like ordinary NPCs in voicechat.
posted this on my tiktok was before so if you've seen them on there, that's me (if anyone somewhat stole them, which i doubt- my pfp is Movie Ambrosius.)