I don't know when or if I'll ever get to work on this idea, but I wanted to present it here before I leave for my new job, so at least the idea is out there. The idea is not fully realized, I only had it a few days ago. It is a game similar in style to Valve's Portal or perhaps even harkening back to Lost Vikings. In short, it's a puzzle mod with (likely) no enemies.
There's a series of "weapons":
A portal gun that operates much like Valve's Portal
A translocator ala UT
A"boost pack" that allows you to jump higher
A freeze weapon that can extinguish flames (wall torches, flares), when fired into the water, it will create 'ice berg' platforms.
A flare gun that can illuminate an area.
The power hammer, that can smash certain obstacles and perhaps give you a slight boost if aimed at the floor.
And perhaps other items, I'm not totally 100% on the above list either. Each 'weapon' uses one of four types of ammo, the weapons and the ammos can be found as items on the level, so this will greatly rely on custom maps.
Dark Ammo: Operates portal and translocator weapons. Stepping into the light will instantly deplete all your Dark Ammo.
Blue Ammo: Operates the freeze weapon, perhaps another. (Air fist?) It is instantly depleted if you fall too far. This allows for Hurried Descent like sections.
Red Ammo: Operates the boost pack and flare gun. It is instantly depleted if you touch water.
White Ammo: Second rarest, next to the Dark Ammo, operates the power hammer and perhaps another. Instantly depleted if you enter the dark - thus you cannot have both Dark and White ammo simultaneously.
When "instantly depleted" ammo of that particular type may respawn or may trigger an event of some type - to allow you a second chance at a particular puzzle. The light based ammos will operate using DP's getlight function.
Puzzles may include a room with a crack in the roof that allows sunshine in a particular pattern on the floor, you must use the translocator to cross the bright portion, and later you may return and are required to follow the light on the floor to retain the white ammo you just collected.
Also moving platforms, funky animated lights, other jumping puzzles and timed/impending events will prevent it from being too stuffy a puzzle (it would get old fast that once if you solved the puzzle of the level, there would be no further point to play it).
Red and blue ammo mostly allows you to retain the others types:
The freeze gun will allow you to extinguish certain wall torches (switchable lights) that will then allow you to retain your dark ammo in that area. It can also create platforms when shot into the water, allowing you to jump across and not touch the water, allowing you to keep any red ammo you may have.
Similarly, red will offer the Boost pack which can soften your fall retaining the blue ammo you have, or operate the flare gun that will allow you to keep your white.
I'd like to get all 4 ammo types two weapons each, but as it stands, it's a rather tight system already. What do you think? Comments appreciated.