I always thought security cameras seemed like a neat idea, but never really had an idea of how they could be implemented. Lots of custom 3D rendering logic, I'm guessing -- do you have a plan for implementation? I can imagine that, in addition to being pretty cool, such a thing would open up great new possibilities for puzzle maps (as well as, of course, PvP builds).
My own modding has mostly been for little things I wanted... a glass-textured door; a "spotlight" that can be used to illuminate an area but is itself invisible and intangible. The most ambitious idea I've had is for a Geographic Information System (GIS), which I imagine as a sort of interactive in-game atlas interface which would get filled out by feeding maps to it. Perhaps the job of mapping could be streamlined through the use of aerial drones or satellites. The trouble is I imagine there isn't much incentive to make or use such a thing, with plenty of good mini-map plugins and external map generators out there already. The only thing such a mod would contribute is it would make the mapping functions part of the gameplay, rather than a feature that's available for free.