In Platinum, my Dialga had Adamant nature (+Atk, -Sp. Atk), and my Palkia had Modest nature (+Sp. Atk, -Atk). I don't think that it's a coincidence. Will Arceus have a specific nature?
You can't get Arceus anymore since its event has passed (unless you hack).
Most Pokemon do not have a present nature, legendaries included; exceptions would be N's Pokemon. You just got lucky and got a Modest Palkia (I wouldn't say that Adamant was the preferred nature for Dialga).