No, Team Plasma basically goes down the drain and doesn't play any major role anymore, though you can battle N, the Shadow Traid and Colress as well as some other trainers like Benga and Alder.
Legendary Pokemon-wise you can go catch Reshiram/Zekrom, Kyurem, Cresselia, the 4 Regis, Latias/Latios, Heatran and the Lake Trio. After catching Kyurem you can also use the DNA Splicers to get Kyurem-B/W, Marrilands has an extensive walkthrough on this
Apart from that you can go around Southern Unova now (You know, the first couple places in B/W which you didn't go to in B/W2 like Nuvema and Accumula town). You also get Black City / White Forest, access to some new Pokemon I believe, and have fun at places like the World Tournament.