I usually start with some sort of a theme, then look at the fabrics for it. I've just finished a couple of baby quilts, and used panels for them, with small surround blocks and frames. They turned out well.
I've also made quilts for family members, and go with colours the person likes, but I choose the blocks and the design.
I'm not quite sure what I will do next. I've got a couple of "themes" in mind, for example an "Out of Africa" quilt, and I have collected quite a few fabrics for this, including some animal panels, but have not yet decided on a design eg blocks and borders. I think this is the fun part, and I spend quite a bit of time with pencil and paper, and then with my quilt design programme. On the other hand, sometimes I just want something really quick, and do what I call a "melting pot" quilt with easy blocks and any fabric I can put my hand on. Hmmmm, sounds a bit contradictory doesn't it, but still, each to their own!! I'm sure you'll think of something really great.