The 3DS has demonstrated its capacity to scan the real world with its cameras and insert virtual characters into that world. In The World Ends With You, Neku scans the world for monstrous Noise. Why not have an option for the player to do the same in a sequel? Players could use their cameras to hunt Noise. 
 
I had a similar thought for looking for Pokemon, but Noise hunting has an even stronger thematic link. 
 
I'm not saying that an ARG should be the only way to play a sequel, but it could make for an interesting add-on.

I would only want that as some add-on that has nothing to do with the main story or something... I think. I dunno, if they can incorporate it well, I'd be all for it. It would be interesting if they let you take a picture of a pin you have in real life and add it to the game, attributing powers to it depending on how your pin looks.

Pokemon Snap in real life.

I want to run around downtown staring at my 3DS hunting noise. 

It was a good game, but in my opinion the story doesn't need a sequel so I don't think they could pull it off

That would be such a good idea for a side game! Unlock some badges by battling noise in the real world with your 3DS. I very rarely say this but brilliant idea.

Fuck no.  Nomura shouldn't waste his time with this.  Let him focus on V13 and KH3.

I think using StreetPass and the pedometer would be really cool for a theoretical sequel. They did really cool stuff with the badges in the first, it would be cool if they could incorporate that stuff with the aforementioned features.