I am also working on a set of variations and mash-ups on themes not written by me (from a famous videogame). I am using so many themes at once that I am also feeling unmotivated lately to work on it. It seems like one of those never-ending snowballing out of control projects. I hope that I'll finish at least one part of it.
I think because of my ginormous orchestral project, the next thing I'll want to write is a piece for 1 monophonic solo instrument accompanied by 1 polyphonic instrument (like the Dreamscapes contest rules). It would be a nice change of pace. And if I manage to write something "dreamy" then it will be all the more interesting.
Besides that I was thinking of writing my own variations of more of the themes from the "Bits to Bangers" competition, since I already did the "Harvest Moon" Piano Quintet. I think I majorly prefer working on previously written video game themes, and if not that, then I prefer for my own music to retain a VGM style or media purpose to it somehow.