Weren't there Pokemon that were only obtainable in Platinum and couldn't be sent back? Because if so, that throws the whole backwards compatibility thing out of the window. Think of it, sequel implies "years later" - and who knows, in the same region they could've discovered a few more Pokemon. I mean, it seems totally logical, and knowing Game Freak, there could be a few Pokemon they were hiding from us, and the fact that the hidden/unreleased Pokemon are pretty much being released much quicker than Sinnoh's makes me think that they've got more up their sleeves and this isn't all we're getting.