Stop right there, the Battle Frontier has been confirmed to be on Platinum, get your facts right then return here.
*Sigh* You mentioned a Frontier Brain owning Heatran, with his or her facility being located within Stark Mountain. The Battle Frontier has been confirmed to be ON THE ISLAND, with no specific locations mentioned. What you did was speculate that Heatran will have an anime debut instead of movie debut, it will be owned by a frontier brain, and that said frontier brain will have a facility in Stark Mountain.
Second, who says cresselie is not on the next movie? The same as Arceus?
Heck, look at this movie!
It has:
Dialga, Giratina, Shaimyn, Regigigas
Of those, it has been confirmed that Regigigas had the minor roll of saving some villages from being crushed by glaciers. Its' role in the movie was minimal at best. Dialga is there as an adversary for Giratina - it does nothing special at all and is cast in an entirely different light than the almighty end-all pokemon of the 10th movie, judging from the previews. If that doesn't convince you, then allow the title to do so. It includes Giratina and Shaymin, proving that THEY are the focus.
And premiers:
Giratina's new form, Shaimyn's new form, a new world.
Why can't the next movie have those legendaries?
I'm not saying that they can't, but if so many legends are jammed into one movie, they tend to get less spotlight. The writers didn't do well at all with Kyogre in the 9th movie; Regigigas' appearance, as already stated, was nothing compared to what the trailers demonstrated.
Heck, Arceus does not even need to be in it the entire time, it can just appear ten minutes before the end and bring everything to a super climatic conclusion, then leave.
So that's it? The pokemon that Arceus supposedly created get all of the spotlight while it gets the shaft in the end, never to be spoken of again? Nintendo is not going to leave what is considered THE top pokemon in the pokemon universe as a 10-minute add-on in the end of a movie. Add in the fact that it is a Sinnoh event, and that proves that it will have the majority of the screentime to itself.
Kinda like the giratina castle in the last movie.