Cressalia could be, it fits theme of Dialga and Palkia
Darkrai can't be, it's an event pokemon
Shaymin also event
Giratina could be, it also fits theme
Arceus also event
HeatRan doesn't fit theme
And of course the three thingies, (Uxie, Mesprit and Azelf) can't be, they're 3, not 1
And, you're...