You can change textures without too much of effort (but not how they are animated). Maybe soundtrack. Otherwise, the game engine is not built for easy modding. Changing anything in combat or changing the way encounters behave will require coding.
There's a built in system of translating the text and interface, which technically gives you opportunity to replace things that characters say however you want. While everything else, like the number of dialogue windows, won't change.
To summarize, making a mod that gives your characters tattoos or new clothing, but similar to existing, is possible, one such mod exists called Redhead. Writing your own version of stories is also quite possible.