Idea 1:

To fit in with the Twin Stick theme i would like to suggest we make a Tower accension roguelike. This is to keep a small scope that can easily be expanded upon in any direction. You can start with a room and make it so enemies spawn for a few waves, and then once the player beats the wave they get an upgrade choice and get put in another room and repeat the process. This can be expanded in many directions by adding more enemies/upgrades/rooms/traps etc.

Idea 2:

Idea 3: