A couple of ideas, late to the game

First, I would create a space dog-fighter with realistic physics, like Babylon 5, but with completely modular fighters and tons of options for customization.

The other game I would create would be like Mega Man or MMX, but without the Wily/ Sigma stages. Just gallons and gallons of Robot Masters to fight. Maybe release it online and have a new RM to fight every week or every month and the options to fight bosses again or turn off upgrades and such. Yeah. That'd be a sweet game.