Overall a nicely written concept document with good detail and of course it is awesome that you've already made a start on development. There's probably nothing overly unique about the idea that I can see, but that's fine for this unit, especially since making this style of game fun and featured will be a nice challenge that I am certain you are up to. It will be important to not get too bogged down in making systems for bullet hell and item/upgrade systems too much that you lose sight of "finding the fun".