Crystal-type Pokémon. They have them in the TV show AND in the TCG. Why don't they have them in the GB games yet? If they were to be real, I would expect them to appear extremely rarely, like Shining Pokémon, only with a Steel Wing glowing effect and some kind of oddity in its elemental types.
I would also like to see an end to the Gym-style game plot. It is getting somewhat stale, and having bosses in the game have monsters with the same type (all water Pokémon, all rock Pokémon, etc) makes the game too easy. I liked XD and Colosseum because the plot is different and the battles are much more difficult.
I think the odds of either of these elements being introduced into the game (or changed) is much worse than me walking outside my house and then getting hit with an impact event (asteroid, comet, falling cargo from an airplane, etc). Oh well.
Postcount = 10^2. Powers of ten rule! Wonder when I'll get 10^3...