To add on to that, that's why we have D/P/Pt. I'm sure that once you get the National Dex(which I'm expecting to be around the time you beat Johto's Elite Four. Possibly.) you will be able to trade with D/P/Pt. For the others, I think there will be a new area. My hopes are now geared on seeing Hoenn added into the game :D(which would be one step closer to that game we all want where you can go to every region. Save for Sinnoh, which again, we have D/P/Pt for).Oh well, you were more descriptive.
I somehow doubt we'll see much Sinnoh Pokémon at all. After all, look at FireRed and LeafGreen; the only Pokémon from Hoenn without trading are the ones that bred to make third-generation Pokémon, such as Wynaut. Likewise, we'll probably see only Sinnoh Pokémon like Munchlax.
But you know what else? We'll probably be given a place to catch Hoenn Pokémon. The Sevii Islands served that purpose too, after all.