Cronus242

I don't mean to sound like an idiot, but I am generally ignorant when it comes to understand just how much can be done in terms of mods. I had an idea for a mod and I had a few questions regarding as to how I could possibly tweak the idea to be better. So, I suppose my question simply stands as: Just how much can be physically done in a video game mod, even in terms of changing game play, entirely?