There really isn't criteria for requesting piano sheets on here, but I think you'll find games on Nintendo consoles are most likely to be fulfilled, especially from newer game releases (at least that's my attitude when I see a request).
I wouldn't say there's no chance of non-Nintendo music you could request (there's a decent number of people who arrange music for games on PC/Playstation) that would end up being arranged, but definitely less likely
Least likely to get fulfilled though would be fan game music because those aren't elligible to eventually end up on NinSheet itself
Hope that helps, what were you looking to request?