It would be nice to be able to flip builds upside down. For example, I wanted to put a gear on the ceiling that was built for the floor.

This could be integrated into the mirror or rotation button. Alternatively, it could be its own button above or below the mirror and rotation buttons.

I 100% agree. I made the mistake of designing a wall pattern on the ground, flying above it for easy view. Now it's useless because I can't actually place it on a wall, so I ended up print screening and copying it manually anyway.

For a short-term workaround, I would recommend worldedit. You could select the area, copy, then 'flip up' and paste to create a vertically, and you can also rotate it 90 degrees on either x or z to make it stand up. Then create a schematic of that for building.