From what we know, Tri Force Heroes is most likely going to get DLC in the form of two new Drabland Areas. But what could they be? The eight existing areas already cover most typical Zelda environments except for a swamp and an ocean area. You can even see some ocean in the background of the map. However, this would be quite boring. We're talking about DLC here and if Nintendo wants people to pay for new areas, then those should better be something special, something interesting.
Something like the Realm of Memories and the Hero's Trial, which both got added in the Anniversary Edition of Four Swords! GREZZO was making that game as well, so it's not unlikely that they might get similar ideas. And I would love to see some retro levels, where they could remake some classic environments and dungeons. This would even be ideal for a late February release, where we're celebrating the start of the 30th Anniversary.
And since most people will have all the overpowered costumes by the time, it might also make sense to amp the difficulty for an additional super challenging area. We already have enemies like the Soldiers that can bring back this special nastiness of the Hero's Trial... This would also be the perfect area to add a shield as an item, since you want to be able to reduce the damage as much as possible.
On a second thought, an ocean area could also be quite interesting, if it adds content from The Wind Waker and Phantom Hourglass. Somewhat of a "retro" area so to say. As long as it's something special, I'm fine.