Based on (what little) I know about modding the game, making new stuff isn't really a big deal (which is amazing, actually). Editing and duplicating XML is easy.
So adding a new bow, and calling it a gun is a good start. Then you'd need a model. I'm not sure about that part. I'm still trying to figure all of this out. 
Making industrial age techs would probably just extend the tech tree. I think magitech would be a neat idea, like steam engines powered by fire elementals and stuff.