Re: Pokemon Dash, Pokemon Diamond & Pokemon Pearl!
Pokemon Diamond and Pokemon Pearl are both based on Light pastel colors, correct? So what if they introduce a new Light-type Pokemon, and that is the themed Legendaries on these new games? That's my theory, anyway, because I've been highly suspecting that they come out with a Light type, to counter Dark type.
The third game that comes out later (I.E. Emerald, Crystal, etc.) I suspect to be Pokemon Platinum, which will probably introduce the first ever Metal-type Legendary. This, of course, is pure prediction by me, I don't have any factual knowledge of this.
Now, before you go saying "Hey! Platinum has nothing to do with pearls and diamonds! J00're crazy!", consider this:
Pokemon Gold and Pokemon Silver both came out, with it's third title being Crystal, which obviously had nothing to do with the previous metal-oriented names. That is why I suspect the third title to be Pokemon Platinum for Pearl/Diamond.
Anyways, those are just my thoughts.