The idea of an elemental puzzle game fits very well with the theme of the jam. All of the ideas feel like they syncronize great one with the other.
But now, reading you design doc, I got spoiled on a twist later on in the game xd.
Do you have a plan of how many levels will be created? I noticed there was no section of things to potentially cut in case of a time crunch. While it's nice to try and keep ahead of schedule, it's also important to have a backup plan in case things go south