I wouldn't count a plane out of the equation seeing as how things have went so far. Spirit Tracks, I didn't worry about. New technology in the Zelda series has always been around, its just been in different things like house design or weapons that Link has, and no one has really cared about any of that. Actually, the amount of technology that the Zelda series has already is pretty advanced, even by our standards. Gamers seem to take notice to vehicles especially in the series, probably because they are closer to what we recognize as technology and advancements, based on our own world. One thing to remember though is that its Zelda, and it doesn't take place on our world. Even if the series went way into the future, or into space or something, there would still be a bow and arrow, a sword and shield, some rupees laying around, castles, princesses, heroes who look the same as the last hero, etc. No matter what Zelda does its always going to have these key elements that make it feel more like a Zelda game than anything else. Phantom Hourglass didn't feel "advanced" in technology; it just sucked. The game was bad so people probably thought that the steam-engine boat was what did that, but I think Spirit Tracks proves that theory very wrong. ST felt very much like a Zelda game to me and as long as its got a good story going on, that's all that is important.
So, don't be too afraid of a plane in the future. There was already a hot-air balloon that Beadle rode around in, so its not like they are avoiding aircraft. I think its possible and I'm not going to say that I want or don't want it, because frankly its not a big concern of mine. If they have one, then its still got a big chance of being a good game because its Zelda.