As the title describes, this post is full of New Super Mario Bros. 2 spoilers! If you are not interested in seeing the final level and final boss, leave this page immediately! For those that are interested, continue on and watch the video!
Very interesting indeed. The main game seems to end at World 6, leaving a supposed three extra worlds for players to find and explore. The Bowser boss battle is as you would expect, however, Bowser has apparently found a few spare hammers to throw around. A second phase of the boss battle brings Bowser back to life, but in giant form. Very similar to the giant Bowser of New Super Mario Bros. Wii, but Bowser is now in the background and the goal is now to move up. Defeating Bowser allows you to save Peach and let the credits roll.
UPDATE: We now have a video of the actual video and the Koopaling battle before the Bowser. Just like the previews showed, Mario will be running through the level as the Koopalings fight him in the Clown Car. There is also a short section right before the Bowser battle where Mario must avoid the incoming fireballs.
UPDATE 2: Higher quality video of the level and final fight: