Magnetic boots and a jetpack is a must, you could have try integrating an fps with 3 dimensions instead of two. That is instead of just x and y, but also include a z. So instead of you're player being limited to forward, backward, left, and right, they could go up and down as well. And no normal fps games like hl2 don't really have that freedom for up and down. The game that did this the best in my book was
Axis by Jamie. It's a fps where you build you're own mech and stuff, but you're never walking, you're always boosting around everywhere utilizing the x, y, and z dimensions freely.
Of course i'm not saying use the axis game engine, or make the fs fps fast paced like axis, nor have so much freedom in all dimensions. I'm saying you could easily get some ideas by checking it out.