I think the Shadow Pokemon were a great addition. I'd love to see another game involving them, but, like others here have said, the idea of catching another person's Pokemon made me feel kind of... evil. I realize the people you're stealing them from are evil, but I don't think stealing Pokemon from people who steal Pokemon is a good way to show that stealing Pokemon is wrong. That doesn't compute for me. Either way, yes, the return of Shadow Pokemon is a great idea.