There seems to be conflicting opinion regarding exactly how moddable RTW is. But indeed, just look at the Napoleonic: Total War mod to see what the community is capable of nonetheless.

There's also an interesting Three Kingdoms fantasy mod being worked on for RTW right now. They seem to be making good progress already, despite the concerns over how hard it is to actually mod for RTW. From what I've heard, modding RTW is tricky, unintuitive and unsupported at present, but it's clearly doable. I guess we've some really talented fans :)