Scrolling the levels is a slick idea, though it does feel cramped due to how far down BONUS ends up combined with the early termination of the screen kernel (it looks like the visible portion of the screen is only 170 scanlines).
Yes, the scrolling window is only 3 floors (girder + corridor) plus 1 girder high (3*32+8 = 104 lines). I'm currently out of resources to add another floor, but will revisit it when the rest of the game is there.
- RAM: only 2 bytes left in ZP. I can probably move the sound variables to SARA RAM, but I first need to have the enemy AI done before I know how much space I would have for that.
- ROM: another floor will need another subkernel in the display bank, which will limit the space available for graphics. I will see that when the levels are done.
- Cycles: The enemy AI might not fit into vblank + overscan and may need to be in the black area. But not all enemies move every frame, so maybe that can be timescliced.
Not impossible, but better not hold your breath for it