Since this page list Keldeo's Pokédex number as 153, I have strong doubts about starting in a new region. I may be overanalyzing, but the page basically promotes using Keldeo in B2W2 early on in the adventure (that's the reason its level is so low), so I suspect that the Unova Pokédex will still be used in the beginning. Granted, there should be a Platinum-like expansion, but it doesn't seem likely that there will be a new regional Pokédex (they wouldn't want us to know about it now, but they could have used the national number).
What if the new region were handled similarly to the Sevii Islands, but better? We would be able to go there before the end of the main story, but not from the very beginning. Imagine if we had been able to travel to the islands as soon as we arrived in Vermilion instead of waiting until Cinnabar.
What if the new region were handled similarly to the Sevii Islands, but better? We would be able to go there before the end of the main story, but not from the very beginning. Imagine if we had been able to travel to the islands as soon as we arrived in Vermilion instead of waiting until Cinnabar.