I mean, multifandom is... doable, but you're basically allowing people to bring whatever characters they want along with quirks and identities that might be closely tied to the fandom said character belonged to. For example, in one camp, you have a vampire slayer slinging his iconic silver revolver, bullets and all, because that's a sure way to kill a vampire in the world he was belonged to, but in another camp, you have a vampire who takes a silver stake like it was a small annoyance, the presence of the vampire would make part of the vampire slayer's identity moot. I can imagine that handling the difference in expectations would require a lot of effort from the GM's end. 

That being said, I speak about... well, probably the worst case scenario. It's your RP in the end of the day, do not let your fun be compromised! 


In another note, what kind of vibe would this adventure be? Survival horror? Survival cozy with fanservice? (Don't mind if it does XD) We have beaches, forests, lakes, and all kinds of terrain for fun outdoor activities... or maybe not, and we should expect hypothermia today and heatstroke tomorrow?  

Will there be any pointers in this adventure? Or it will be a giant sandbox?