This is an awesome little game! Are you or could you make another one with different mechanics?
I think there's a lot of design space that can be explored with this set of mechanics, and another boss could totally be feasible (in a separate cart, since this one maxed out the PICO-8's capabilities). But I'm probably not going to be the one to do that. I have every intention of making more boss fights (I think boss fights are a beautiful game design challenge) but they won't use this set of base mechanics or be in this "world".