It's pretty much become a genre at this point, hasn't it? I used to believe innovation stopped being a thing recently, we'd already threaded all the possible ground on gaming, so it's pretty cool seeing history happen before our own eyes.
I wasn't really planning on making this engine originally (conceptually it's not very challenging stuff going on behind the scenes) but I experimented a bit with making my own game for a jam and figured "heck, I've got some cool code here, might as well clean it up a bit and turn it into an asset". The biggest challenge really is GM's performance, the number of enemy-to-enemy collisions increase exponentially as the game goes on and it makes things go from smooth to unplayable very quickly once you hit the sweetspot.