I'm glad that everyone seems to be enjoying it. The response has definitely been overwhelmingly positive, which makes me really happy.
I've been making some good progress on the sequel over the past couple of weeks. I'll be using a mapper instead of NROM so I have a lot more room for varied graphics and new gameplay elements. I've already added in ladders, springboards, and hidden doors. I'm hoping to improve on the boss battles too as I felt they were a bit lacking in the first game. Also thinking about getting rid of lives in favor of a death counter and speeding up the transition between levels, maybe treating a block of levels as rooms within a larger stage. It's all pretty early so far, so who knows what I'll come up with.