My idea for a theme was mistakes.
Like,
Every stage has his own "little mistake"
Like on stage 1, everyting is huge, like you made your stage normally, but then you made It giant by accident.
Or liek stage 2, everything is turned around (trigger_push pushes you up) and the ramps are rotated like 180°.
Like in surf_funhouse a little part.
Another stage that the textures are rong, like : when you make a skybox in hammer, it doesnt look like the sky but liek a light-blue box with "skybox" in it very much.
Then, you should make a texture for css that looks like it and use that as "sky"
Also teleports or something, With a "trigger-texture" That the people like see the map how you see it in hammer.
And ofcourse a few more "mistake-stages" but these were my ideas :p
I don't know or it's a good idea, so i taught just to post it so you guys could rate it.
I think it would be fun as tfm-map.
~ Mr. Blup