I'm using the runtime to make an abstract (Rez meets 20th century modernism) first person action game with a non-representational visual style. The fact that no pre-existing weapon & gameplay code whatsoever is in the runtime is actually a blessing for me, because I'm not creating a by-the-numbers FPS.
Personally, I'd love to see the runtime used to broaden the horizons of what is possible with 3D games. UT2003 is a pretty darn good base to mod from, but all of the mods you see for it (stuff like Marble Madness 2003 excluded) bear UT's stamp heavily... there's X weapons, you run around, jump and crouch, etc. I don't even need to mention how many of those mods harbor pretenses towards "realism", which in the end means that they're all going to be about humans running around (at slow, "realistic" speeds) shooting each other with instant-hit, highly lethal weapons in some kind of modern military setting. YAWN.
It's much more of a daunting prospect to strike out on your own into new realms of game design, but it needs to be done. The game industry is drowning in a sea of mediocre me-too titles and it's up to indie developers to shake them from their complacent sleep. The runtime seems like a good place to start.