No, I think this would destroy the advantages of a 2D (or 2,5D) Four Swords game, especially when it comes to level design. One of the best things of the first Four Swords game (on the GBA) were the randomly generated levels. This was really cool and added something to the replay value. We're not at the point, where randomly generated 3D content can be done in a way, that doesn't look cheap. The second advantage is the possibility of an easy level editor. User generated content becomes more and more popular and I could imagine, that the multiplayer mode of Spirit Tracks will have a stage builder like the one in
SSB Brawl. Since the 2,5D graphics on the DS aren't as advanced, you could easily build levels, that look exactly like the original content of the game (unlike in
SSB Brawl, where the visuals of custom levels were pretty bad compared to the rest of the game). A multiplayer game lives from new content, randomly generated levels as well as user generated levels are important possibilities for keeping a multiplayer game alive. And I believe, if Nintendo creates more multiplayer experiences for Zelda, they want to use these advantages and therefore 3D would be a bad choice, because those feature wouldn't be simple enough or maybe even not possible with 3D.
Multiplayer games have to look simple to be fun. Why do you think Tetris and Wii Sports are wildly popular? It's the same reason, why Nintendo now makes with
New Super Mario Bros. Wii the first 2D Mario game on a console since the SNES era, instead of just adding a co-op mode to
Super Mario Galaxy 2 and advertising this instead. 3D is not always better. Because 3D makes games more complicated. And I believe the same applies to the future of multiplayer gaming in Zelda. Right now the Zelda team focuses on adding multiplayer Zeldas to the DS and I can't wait, how the mutliplayer mode of Spirit Tracks will look like (hopefully better than what we got in Phantom Hourglass). And I believe, if Nintendo does a new Four Swords game on the Wii, it will be 2D or 2,5D and they will advertise it as a simple and fun Zelda experience, much like New SMB Wii.
And I would support that. I couldn't get any of my friends of playing a complicated 3D Zelda with me, it has to be simple and addicting. Like Wii Sports, or Mario Kart or New SMB Wii. Therefore a 3D Four Swords game wouldn't be of any use for me, because I wouldn't have anyone to play with (except for online maybe, but bah... online gaming is never as fun as local multiplayer).