Like I said before, Square can't make a
sequel. Only a game that compliments the first. Also I'd like to note that TWEWY did well in America because it offered insight into modern Japanese culture. In TWEWY 2 it might be a good idea to do the same thing for another country, it'll be an intresting concept for the game to be set in another modern influenced area like Paris, London, or Barcelona.
I... don't think so, actually. Half of the character of the original TWEWY is the fact that it was set in Shibuya, the center of Japan's youth culture. If there was to be a second TWEWY, the designers would have to not only find a new city with its own youth culture, it would also have to be an area heavily dominated by shopping, etc. Besides, The World Ends With You was basically built around Shibuya. Trying to change locations would be like trying to switch a house's foundation without moving the house.
Me? If there's a second TWEWY I think it should be a prequel giving us insight into the lives of some of the Reapers or Joshua. While it's established that Megami, Ram-Crotch, and most of the other reapers have been in the business for a while, we have no idea how long Sho's been in the business so there's room for an origin story. Likewise, it might be fun to see how Joshua became a composer. All of these people had to play the game at some point, so it would be interesting at the very least to see how they played through it and why they made the decisions they did when they won.
If there was going to be a continuation of the original TWEWY.... it might have to be centered around someone taking Joshua's place as the Composer and trying to screw with Shibuya. Neku and the others might or might not get sucked back into the game when they try to figure out what's going on. Either that or it could give us some insight into the angels and Hanekoma.
There's enough questions that can be raised about TWEWY that I almost think there's room for a sequel.