Personally I think it's much different to that, I mean more in terms of the story not the users access to the opposite Pokemon.
For me I bought White, so i'll get White 2, in this game, we all know N got Reshiram and ultimately flew away at the end to a far away region, is White Kyurem 'N's Pokemon? Or are we going to be the one to catch it.
I think that Kyurem being in White 2 and vice versa is definite evidence for the cover pokemon being the Rom/Ram that N caught in White and Black 1. It makes the most sense in terms of not sticking to the rule established by the first two is having the legendaries on oppositely coloured games.
Other things I want to see from the new games:
Pre 5th gen pokemon available in Unova
Fights with the Shadow Triad/Sages who have escaped from custody. It may even be the Sages as the main villains, they could have perhaps tracked down N and taken his dragon from him and discovered how to create the new form of Kyurem from the captured dragon.
Starting completely afresh with a new character in Nuvema, but encountering the old characters along the way - I particularly like the idea of Bianca working alongside Professor Juniper, not so keen on her being the professor herself - a lot of time would have had to pass.
I also really want the battle frontier to return, although I think that it is probably a given.
A more farfetch'd idea would be to be able to connect to your own Black/White game and somehow download your first Hall of Fame team, meaning if/when you fight Hilbert/Hilda in Black and White 2, you fight your own team from your original games, not at the same level or even same movesets, just the same species.