Darkrai will not star in the third Sinnoh game because it is an event Pokemon. When have they ever had an event Pokemon be the star of the third game? Or any game? Never, that's when.
According to Wikipedia, Giratina's name is based on Girasol, the fire opal.
Now that I think of it, having a game based on Giratina would be quite odd, since Giratina isn't part of the Sinnoh Pokedex. You need National Mode before you can go capture it.
But if you recall WPM's random pokemonopal.com check that led to Nintendo's official site (go ahead, see for yourself), and consider where Giratina's name comes from, we very well may see a Pokemon Opal Version within the next few years.