This is a really impressive demo, and I enjoyed exploring it all the more knowing the technical details that you've shared on LostCast over the past few episodes. I think I explored the entire dungeon twice, but each time I came to a final door for which I could not find a key. Hope I didn't miss anything.
What do you think you would add next? Just tiger hatting here, but I assume you could render floor tiles as the GBA does its rot/scale backgrounds: in horizontal lines across the destination texture, offsetting the read position in your floor texture by a dx/dy vector that is set based on the rotation of the camera. You would also need to change the initial read position, as well as the dx/dy values, at the beginning of each horizontal line.
Phew. Tiger hat off.
I would really like to see you continue work with this prototype. I'm sure we'll hear about it on LostCast if you do!