They've never done an event Pokemon as a mascot of a game, also Mewtow was created by team Rocket
and It didn't have a plot to the main storyline of R/B/Y.
The god stone will change Kyurem's form, the15th movie plot tells that Kyurem and the Swordsmen; battled in the mine in the mountain in the past, which created an Ice Dragon Gate in the process of battle so they could battle on the other side.
I think the ice dragon gate will make an appearance like the Distortion World.
No....it doesn't. God Stone is just an image that was left over from beta testing as far as we know. The original Concept for both the Light and Dark stones. THAT is the ONLY proof the games have. There IS no canon relation of the God Stone and Kyurem NOR is there Canon Proof relating Kyurem to Zekrom or Reshiram.
Kyurem's Movie is him vs the Musketeer Trio. There was something said that they had fought once before, but that may have been fan translation mixed in. As for the Secret that is sealed in Kyurem's Ice. Its Keldeo. Why else would the Musketeer Attack Kyurem?
Not until Giratina's Movie did anyone have solid belief that Platinum would have Giratina as its Mascot. Giratina, IN ITS MOVIE, was shown to be related to BOTH Palkia and Dialga. So do not state your personal opinions as a fact.
Colbion has an actual background story that can be exploited. A Better Background story imo than Kyurem who at this point is just a Dragon Version of Deoxys in how it came to be where it is. There is a lot that can be done with Colbion, treating it like Suicune and the Legendary Beasts, having it repeatedly run into N or the Protagonist at various points. Including the Moor of Icarus where the great fire that cause the musketeer Trio to start hating Humans.
Colbion, Virizion, and Terrakion share more personality traits with N than any of the other Legends, and N would most likely be able to relate himself better than with a beast who only wants to kill and feed.
Who the third Mascot will be is still any Pokemon's Game. Don't state Opinion as fact. Kyurem has not been proven for anything and the movie coming out is only HALF his. The Other half belongs to the Musketeer Trio.
Also, I'll bring up that there were members here that supported that Youtube video that "Proved" Samurott, Pignite, Emboar, and Serperior were "Fake" as well as various members that were 100% positive that there would be a "Light" type added into Gen 5. Both were proven wrong. Do not accept personal opinion as a fact without proof.
Kyurem "CAN" be "Grey"'s Mascot.
Colbion "CAN" be "Grey"'s Mascot.
Volcarona "CAN" be "Crimson"'s Mascot
Landorus "CAN" be "Crimson/Brown"'s Mascot.
That is speculating.