Well, there is a way, though not like the Gen 4 standards, where they have stronger Pokemons and you gain Exp. from the battle.
In BW2, you can battle the Unova Gym leaders in the PWT. Talk to the right attendant and select "Unova Leaders" to battle them again.
Now, BW2 also presents a nice feature which let's you battle Gym Leaders from previous games. After defeating the Unova Leaders, you will be given the chance to fight the Kanto Leaders, Johto Leaders, Hoenn Leaders, and Sinnoh Leaders.