So engine-building games in playable form are really just "engine system" games to me. The building part is only one way in which you can interact with an engine system, and here I'm using "engine system" very loosely... really I think I just mean anything that exhibits faster than linear growth. But to put it in less mathy terms, a system that creates things that create things. A factory-factory, I guess.
Anyway, I was thinking about other ways to play with "engines," and I started to think about pruning an AI-powered engine after looking briefly but lovingly at my copy of Root. It was a short hop from there to "oh I'll just make a game about pruning a tree" and now here's my 2nd playable, pretty much done!
(I haven't made it playable anywhere yet, ha ha. you'll just have to enjoy the gif :P)