Here's a list of things to do. Note that I didn't write the following, Twilight Raver did.
- Southeast Unova opens up. The early areas of the first games, where Hilda and Hilbert first started their adventure, now contain very powerful trainers. -- One goal is to head to the P2 Laboratory in that area, where the rest of the Team Plasma members seek atonement, including Corless. He can be rebattled once a day. - Clay Road opens up, an area that connects Driftveil City to Twist Mountain. -- Regirock can be caught here, and depending on your version, you unlock the key to catch Regice or Registeel. To get the other, you need the opposite game to send you their key. --- Afterward, with all three Regis in your party, Regigigas can be caught in Twist Mountain. - Black City / White Forest opens up, and the Black Skyscraper / White Tree Hollow challenge is available. It consists of several floors full of Trainers Levels 60+, peaking at Level 80 (presumably 85 on Challenge Mode) with Banjirou, Alder's grandson. These trainers can be fought for experience, so it's a great place to train. - A plethora of legendaries: -- Cresselia awaits the player on the Marvelous Bridge, but only after you have the Lunar Wing from the Stranger's House on Rebirth Mountain. -- The Lake Guardians show up in three different areas of Unova. To first activate them, you need to head over to the Heart Cave on Route 20. --- Mesprit rests atop Celestial Tower. --- Azelf is spotted on Route 23, on the western side. --- Uxie is seen right outside Nacrene Museum. -- Zekrom in Black 2 / Reshiram in White 2 are available. At Victory Road, look for a Zoroark on the overworld map, you may have seen it blocking a cave entrance. It will lead you to the cave, which contains the ruins of N's Castle. You'll speak with him for a bit, then he'll challenge you with the appropriate dragon. After defeating him, he'll give you the dragon's stone, which activates on top of Dragonspiral Tower. From there, you can catch Zekrom or Reshiram. --- And from there, Kyurem is available in the Giant Chasm. - Plenty of powerful trainers for you to face. -- As mentioned earlier, Corless can be challenged daily aboard the Plasma Frigate. -- The Shadow Triad can be challenged in Icirrus City once a season. -- The Striaton Trio can be challenged daily, with one of them teaming up with you against the other two. -- Banjirou can only be fought once only, I believe. -- The Pokemon World Tournament. Gym Leaders and Champions from past regions, as well as the Unova region, all gather here for you to challenge. -- Hue awaits you in Undella Town. Challenge him once there, and then he'll head over to Driftveil City to the Team Plasma building, where he can be challenged again daily. -- Alder can be challenged once in Sangi Town. -- Cynthia can be challenged daily in Undella Town. -- The Game Freak cameo trainers can be challenged daily in Castelia City. -- Finally, under the right conditions via the Memory Link feature, Cheren, with his team from the first games, as well as Bianca with her team from the first games, both based on the file that you link with, can be challenged once on Route 5 and Route 1 respectively. There's also N Harmonia, who has some difficult requirements to even challenge, but he has four different teams based on the season, and can be challenged once a season. They're very powerful teams that take advantage of the weather.