Oh, right. You're welcome :D
I'd use Primal Dialga, personally. You should have changes, sure, but I'd keep it close to the real Diamond. Perhaps...
You could make it like Platinum. Cyrus summons Dialga and Palkia, then Giratina comes out and the Torn World opens. Only this time, Dialga and Palkia go to the Torn World and become Primal Dialga and Primal Palkia. Although, battling 3 legendaries at that stage in the game is a bit much- perhaps have it so you can go to the Torn World after the Elite 4 (you know, like getting the Griseous Orb), and battle them then. If they faint, you can battle them at Spear Pillar, but in their regular forms. That'd be... Something I'm just sorta ranting, so just feel free to ignore or take the suggestion, lol.
Finally, I have decided: the hack will be called "Dark Diamond", and will have a plot which will involve the shadow pokémon. Thanks, again, and now I will edit the post and I will add you to the credits.