Would she and Shadow make the natural progression to romance, or would things get in the way? Such as life, parents etc. Would they realise that they were better off as friends or siblings, or would they form a romantic relationship?

Before Rayne was taken, Shadow had formed a crush. Rayne knew, of course, but she didn't bring it up. If Rayne hadn't been taken, I'm sure the crush would have come out. Mal, Inara, River, and Jayne would not have stood in the way. Though, I'm sure Jay and Matt would have. lol. After that, I'm not sure. They would have kissed, but I doubt they'd ever totally be together.

Maybe the fact that she was gone was a good thing in that respect... it brings in a rather star-crossed, us-against-the-fates kind of feel to their relationship.

Great point. Now that Rayne has been gone for years, it gives Shadow a :'I must protect her and I love her, but it isn't right.' Deal.