Power Brick & Light Strip
My idea is to provide a power brick - much like what is known from power functions sets like the train sets (such as the 7939 Cargo Train) only smaller so it would be easy to build into sets like houses, bridges, vehicles, etc.
To this brick could be hooked one or more light strips that could then be threaded - for instance through Technic bricks - through structures to provide a source of light.
See concept as I imagine it in action below.
For the brick I have used the sound brick from the Alien Conquest sets as a model - I imagine that would be a practical size.
I imagine 2x AAA batteries would be sufficient - I have seen LED fake candles run off 3V lithium batteries for up to 30 hours, so a set of regular batteries should give a good period of light.
Depending on what is the best solution in terms of ease of use, replacement if broken/burned out, etc, the light source of the power brick would either be inside or outside the brick.
This would also be where the light strips would be hooked up.
The brick needs to have an external button or pressure plate for switching on/off.
BUILT INTO ANTHING
As the powerbrick has regular top and bottom surface, it could be built into anything.
A crossing bridge with light strips threaded behind a technic brick - allowing the light to illuminate the walkway.
Adds a little something extra to a walkway :)
Light strips could be 3 sources as far as I can tell from a bit of research.
1) Light tubing: This is known from PCmodding and allows light to spill throughout it's length - is usually illuminated from a bright LED light source.
Can be cut to desired length and is available in a variety of thicknesses.
2) LED strips which of course provides a chain of lights
3) Fiber optic threads - also provides a length of illumination - fairly easy to work with and comes in a thickness well suited for lego modelling.