I've toyed with the idea of an epistolary RP (RP that takes the form of letters) before but haven't gone anywhere with it. I don't don't have a clear enough idea of what it would be and I doubt anyone has much interest in such a weird format. Similarly and weirder an in universe document that tells a story through a series of foot notes by two different editors. 

I've also thought of doing a King Athur story but in SPAAAAACCCCE. I haven't attempted it because I have a very clear idea of what tone I would like to hit and I feel like I would need to be too proscriptive to make it work.

Finally I really would love to do a good investigative horror game ala Call of Cthulhu were the players actually generate the plot as the go along. I've made several attempts at this but they keep breaking down because I lose key players and depend on them for plot points.