I've got lots of random functions that I keep copying from one shader to another. #include would be a great help.

Since this is not necessarily supported by all drivers, I would need to implement it myself.
This shouldn't be too hard, since it's essentially just a basic search for "#include ...\n"

A problem might be that there is less autocomplete support. But that's likely not too big of a deal. I could for example give each file the same name as the function they contain.