Why not DLC-style Pokemon. A sort of buffer of new pokemon between generations. I think it would be a lot more exciting to learn about Kaldeo, Meloetta, and Genosect months or years after B/W was out and to learn that they would be downloadable. It would be a lot better than "hey guys these pokemon exist, and you can't have them, oh but here's an event Mewtwo that can chill with the other three in your boxes"
--
http://img832.imageshack.us/img832/9984/roflbotg.jpg