Pokémon Black/White 2 has something called a “Pokémon World Tournament”. While it isn’t certain whether this is a separate mode or one of the new in-game events, the World Tournament will allow you to fight older gym leaders and champions like Brock, Misty, Blue, Giovanni and others.
Other things have changed, too, in the time between Black/White and Black/White 2. Cheren is now a Normal-type gym leader, while CoroCoro magazine confirms that Bianca is the one that gives you your starter Pokémon this time around, as a favour to Professor Juniper. Additionally, N from the original games has gone missing and no one knows where he was.
CoroCoro also reveals that Tornadus, Landorus, Thundurus have new “Incarnation” forms. Serebii.net reports that the three have undergone no type changes.
Following Pokémon Black/White 2’s lead, the Pokémon Best Wishes anime will switch to Best Wishes 2, featuring more older Pokémon, the return of Dawn and Cynthia, and involvement from Giovanni in the next Team Rocket-related plot. More details from CoroCoro can be found at the Serebii.net link above.