someone can use what the game already has and build from there.
That is essentially what a mod is.
I've look through the code of TM many times never looked at the lighting system.
Lighting is only one part of a shader.
The thing I want most is a shaders mod.
I'm sure it will eventually be possible, but shader mods are a bit more tricky to support.
It's easy enough to write the shader programs in HLSL. But there are a couple of problems.
1. I'm not sure if XNA can build shader programs on the fly. If it can't, then it's going to be tricky to work out how to get the game to load them.
2. A much harder problem is data. If someone wants to build a more sophisticated shader, they're likely going to need to pass the shader programs more data than the game currently passes. And writing a mod to build that extra data could be extremely difficult.