It would be cool if they reworked the music editor, adding the ability to extend notes and controlling their attack and release

I know Castle's email address. I told them to add full screen And now I'm waiting for their answer.

The sixth idea can be possible with using a samsung phone.

Castle uses LÖVE (LOVE2D) and sadly LÖVE only has 2d,doing 3d would mean making a full on 3d engine system and it may have terrible performance.

3D models could use THREE.js (3D html)

Okay these are actually cool