So I'm working on a hombrew setting that takes place beneath the streets of New York City, which uses the d20 Modern system. In this setting, the subway acts as a conduit for generating magic (known as Radiance) in the underground tunnels and caverns (known as the Underworld). In the Underworld just about anything is possible, from caverns that mimic Wonderland, to neo-victorian london streets, toa literal devil living beneath Central Station.
In this setting, regular technology does not work in the Underworld due to the Radiance. Instead, they use what is known as Salvage Tech. A form of technology created from scrap and powered by Radiance.
Salvage Tech is the bizarre science that is the purview of the Underworld. Using junk that they have found in the tunnels, or purchased from traders who have carried it below from the surface, Underworld inventors bend reality to their will, warped through the Radiance – producing inventions that physical science could never envision or achieve.
I'm trying to come up with a system for the construction of Salvage Tech that is structured enough to be used in the game, but loose enough to allow players to create just about anything they desire. Be it a lightning sword that draws power from the subway's third rail, a Radiance powered laptop device or a gun that shoots cheesewhiz to suffocate opponents.
I would really appreciate any assistance or ideas. Again this setting uses the d20 Modern system, which is essentially 3rd edition D&D with a few variations.