Or maybe an event much like the ghost Marowak in RBY and FRLG, were you couldn't catch it and had to defeat it in order for it it return to the afterlife. Maybe in Platinum, you have to defeat Dialga and Palkia for them to return to their rightful dimension.
Exactly! Forgot about that ghost Marowak.
'Course, Shaymin would have to be able to be caught without an event, or else there would be a lack of available legends compared to D/P. And if Shaymin indeed has a new form/type, I have a feeling it will be available for capture .
If this game is actually coming out, the supposed release date in Japan is September-ish.