I loved The World Ends with You, and I'd LOVE a sequel. It's one of the few titles I played through from start to finish without much, if any other forms of media in between (T.V time/movie time/other game time was used for TWEWY), which I also did with Strange Journey, P3P, Secret of Mana, Oracle of Seasons, and I think World of Goo.
To be fair, I didn't understand/couldn't use the top screen from gameplay, so I only controlled Neku, and let the CPU control the partner. If they could somehow fix that, I think it'd be a lot better and user friendly. Other than that, though, I'd love it. I loved the original idea behind the two screen mechanic, the story, the art style, the music - everything. Hell, even the pin smasher mini game was a lot of fun when I played it with my friend's little brother - it was great. And the StreetPass functionality is practically a given - exchanging pins/scores/menu items is right up their ally. I'm all for a sequel. I think it'd be really awesome if they took music from your SD Card and played it during some of the transitioning scenes, too.
Edit
The 3DS is getting both another EO AND Class of Heroes 3D. Class of Heroes was pretty much the PSP's equivalent to EO, but with more party members, and more focus on attack and race statistics. Really fun game.