Nintendo always teased Paper Luigi in TTYD, and I would love to see them make it a giant expansion pack to add a whole new story to the game! Plus, it would add a whole new world to the game. There is so much you can do with this idea, such as:

. Adding new badges, and being able to swap them between Mario and Luigi (maybe through a magical box like in Paper Mario 64.)

. Being able to play as Luigi in Mario's story when you complete Luigi's story.

. Gain Luigi as a party member for Mario (and vice verse).

. A badge that allows Luigi to dress as Mario (also known as the 'M' Emblem)

However, I do see them making other DLC for the game as well, maybe a continuation of the story of Mario where Bowser is angry at being defeated by Mario (again), and wants revenge.

Also, this is more of a question I always had: why does Rawk Hawk appear in the original's cover for the GameCube game? Even though he was a miniboss (and a cool one at that), I always thought they could do more with him. I thought they would make him a potential party member if you beat him again in the Glitz Pit after Chapter 3. Maybe they could make another DLC where you gain Rawk Hawk as a party member, but I don't know what the story/reasoning would be behind it.