Do you want to get a taste of all the possibilities or gain experience in one particular niche? You could always do some more modelling and texturing, I'm sure it takes quite a while to be able to get good at that. But otherwise I'd suggest setting a small goal for a mod, like a dungeon with some kind of unique experience, and start working on that. You can decide if you want to use pre-existing resources, or incorporate your modelling/texturing education and make it wholly original. But you'll also have to learn a good deal about using the CS and scripting and such. Hell, I haven't even done any of this stuff myself. But I know that you have to spend some time with the CS and learn things before you can realize increasingly complex visions. Just start out with something small.
I guess what's important, if you really want to go into game design, is to develop an understanding of how players interact with what you're presenting them. I think a dungeon with a "unique experience" could be a good way to test your understanding of player interaction. Use your imagination!