I've tried it in the past, but I realize I just can't get into drawing for someone else's comic. I didn't realize this until I tried it out with a writing team and found it wasn't for me. I still feel bad about having to back down from the project :x but going forward I'm able to be honest with myself and know that I can't muster up the hype needed to work on someone else's script, unless it were a very short (like less than 10 pages) sort of deal. Any other kind of art I'm happy to collaborate, such as on storyboards. But comics are such an intensive process to me, it's hard to spend that energy on someone elses' idea.
As for my own ideas, I have two main ones, but right now I'm putting 90% of my energy into only one of them, with the intention of putting what I learn toward the other, more complex comic. D: But I have had the same two story ideas bouncing around in my head for years and intend to complete them both.