These are the possibilities
manaphy: for one reason, it's part of the sinnoh dex unlike the others. game name mineral, metal
Giratina: its a dragon like the others like darkrai_shadowforce said. game name granite, gravel (mabey opal)
Darkrai: He's in the new movie w/ dialga and palkia. game name ???
Lucario: I dont see him going on the cover, but he is one of the pokemon showed a lot. game name ???
Arceus: It has a like .00000001% of making the cover in my opinion. game name adament, ampthsyst
Creselia: Bigger chances then lucario and arceus but still low. game name carnelian
Regigigass: It's slim for this guy but he might make it game name rock
Heateran: A good pokemon that probably wont make it on. game name Hemitite
These are who i say have a shot at the making the cover(except 4 arceus and lucario). some have a bigger chance then others. one legendary will make it and he might not be on here.
pm me if you have rock names that start with d or L other than diamond.