Two larp ideas that came to me in dreams, a few nights ago. Dreams have often been fertile sources for ideas, although often they turn out to be a bit useless. We shall see!
The first one
So one of these was inspired, I think, by Damon Knight’s short story ‘Four in One‘, which I read when I was I guess around 11 or so, and which made a great impression on me (and has evidently been lurking in my unconscious ever since.) The larp is called The Report on Planet XB325/1 and the characters are a group sent on a scouting mission to determine whether the said planet is suitable for human life. While on the planet, they had an accident, as a result of which one or more of them may have been replaced by an alien slime-mould-type creature which has built a replica body and has access to all the victim’s memories. It will be up to each participant to decide if they were so replaced; and if so, are they aware of it; and if so, what might that mean?
The larp is set on the voyage back to Earth. At any point, any two of the characters (out of six) can mutually decide to expel a character into space, where they will certainly die (whether human or slime mould). It needs at least three to get back to Earth, otherwise everyone will die. There is no way to distinguish replicated people from regular ones. It is not obvious what kind of threat (if any) the creature might present, were it to be brought to Earth. (There’s a lot more details determined in my mind, but I don’t want to go on at great length here. Hopefully you’ll get a chance to take part in it at some point 
the idea is that it turns a slightly fresh eye to the usual kind of existential questions that larp so often addresses.)
In my ideal dreamworld the expulsion mechanic would work via each person having an electronic box of tricks with six numbered switches — if at any time two switches of the same number have been set to ‘Expel’, then the corresponding participant’s box lights up with an alarm and their character is instantly killed.
I’d need to get someone else to make that for me, though — so for playtest purposes I might arrange it via a website instead — or even just a Google Sheet with six tabs.
The second one
The other one is now called (thanks to the tremendously clever Laura Wood, who came up with a way of making it work) The Knights of Queen Tiffany. The characters are actors, who have been called together to devise a play – that is, they are to improvise material together in-character, which eventually will get concretized into a theatrical production. But the larp is just about the preparatory stage.
The play is set in a legendary–medieval world, but the actors are each reluctant to admit to the others that they know very little about the source material, having done none of the preparatory reading that they were supposed to. Therefore the improvised scenes are likely to be rather tricky to navigate. I’m not sure yet what the theme of this is supposed to be (other than it’s about story-making and about teamwork), or what the structure will be – but for now it’s enough that it might be fun to take part in.