Arcade style game.Good scope, re-playable,  lots of tutorials online for places to start. Tough to differentiate your game among the plethora of arcade games out there.

Put all the game on screen, literally. Visualize all the inner workings of the code, the buffer, the event handling, the AI, everything. Put the entire game on screen.

Anti-theme game concept: Game on too many screens.

Force the player to play across multiple screens, maybe browser windows plus smart phone(s). Make the screens a game mechanic. Using a browser extension based HTML5 game you should be able to track window and tab positioning.

I’m leaning towards an arcade game for simplicity so I can take the time to polish and juice it up.