It's a neat idea and I see this in places where you have a generic ruleset and then each setting adds their own rules on top of it.
The thing to watch out for here is combinatorial complexity. The more modules you add, the more difficult it is to work out how the ruleset combines to play together. This shouldn't stop you from building the modules but I think it's a lot of work that is hidden from the initial idea.
Don't forget that to give the system a good run, you need to create at least one setting. I obsessed with this idea, I know, but those systems that have no setting have real difficulty demonstrating the novelty of the system.
Are you going to be uploading and sharing in a modular way too?