The first part would be for threads discussing the current saga (BW), while the second part would be for threads discussing past sagas (OS, AG, DP). The past sagas area wouldn't be for previews/reviews for which we already have a subforum; it would be for topics discussing past characters, Pokémon, etc. that don't pertain to or aren't included in the show now.
An example of how this would be helpful:
User 1 creates a thread discussing Misty's time on the show. User 2 posts in the thread, saying, "Not another one. You know this is going to get closed."
In the past saga area of the anime section, users could create threads discussing such topics without having pointless posts and could actually spur meaningful discussion. This isn't to say the same types of threads should be made over and over again here; it's merely providing a place for users who preferred past sagas to discuss whatever they want about them in a place that's not predominately about the current saga.