Serebii confirmed that they are part dragons. I do agree though that the new legendaries do look cool. They look like something out of an awesome anime. Light type thing might happen though. I personally think it's time for Light vs Dark. It would bring a whole new edge in battling. If they make strong light types, it will be the new thing to counter. But the first guy could be Normal. If there isn't Light type, it would probably be Normal vs Dark, but Normal types are just so useless except a few, I'm betting on Light type. I think that this could be a good game. And on Serebii they also confirmed that there will be further differences between the games other then the pokemon. If the differences are big enough, Like maybe a different side quest after the game and not just pokemon, then it would be even better. Before it was just this. Oh here have a Diamond or a Pearl! The only difference is that both of them have different pokemon then each other which is becoming totally unique! But with this : Oh! Have a Black or White! They have the same base storyline, but the storyline is different from each other! There is different pokemon, but you'll enjoy playing both! Have a nice day!
I just thought of something else. Once you capture one of the dragons, Zekrom allows you to travel back in time or foward in time, or maybe both to see mankinds destruction of themselves. Reshiram could allow you to travel foward in time or maybe back in time, or both to see mankinds prosper/peace.