I agree with Lemon Merchant - if you are building on someone else's work to a significant degree, you should at least let them know ahead of time. If you have a good plan they may well be approving.
I also suspect you are underestimating the size of the project greatly. C2C has been the work of many people over many years, building on top of a game that was built by a large team over 1-2 years (plus another 2 for expansions). I've seen far less ambitious projects sputter out after the creators realize how much there is to do. I might advise instead offering to contribute to C2C, as I know they still have a wish list of features and things to refine.