I can't see it being Landorus at all really, I think we will see the whole Platinum thing all over again and see Kyurem with a new form and it being able to learn two new moves which no Pokemon can have at the moment (Cold Burn and Freeze Shock iicr). I think the new form would have an extra 20 base stats, to level with Reshiram and Zekrom.
I think it's been made obvious that Kyurem will be the third game mascot.
There haven't been any direct references that prove Kyurem will be the third game's mascot or even if there will be a third game altogether, as far as I know of.
It would be awesome If Kyurem's new form would be like this.
I found this image on a Japanese website.
I'm thinking Pokemon Gray and Kyuremu as the mascot. ^_^
And maybe Iris + Shaga double battle for last gym, as they were exclusive to B/W, why not battle both in Gray....? =P Or maybe you get to choose... I dunno. =3