some users will need different functionality from others, and I just want to "require" & call the appropriate bits.

Perhaps a better, easier to maintain, approach would be to put the common parts in to a module. Then write function specific programs that use (or require) the common module.

If function modules already exist, then each function-specific program would just use (or require) whichever function module (or modules) is (are) needed, as well as the common module.