The allure of Zelda is, in part, it's time era. It is a medieval society on the verge of industrial revolution. In games like Wind Waker and Spirit Tracks we are subjected to minimal technological advancement in the topic of transportation. So it's natural that we'd see this idea perused in future games as well. Now that land and sea have been covered, only the sky remains. My naturally conjures an image of Link in a steampunked/Da Vinci style airship/hot air balloon.
However, it's important to look at how air travel would affect Hylian society and even what led to air travel. Let's say this future game in question lies after Twilight Princess in the timeline. One of the most noticeable differences about Hyrule in
TP from Ocarina of Time was how broken the landscape was; ravines as far as the eye could see. While trains would at one point be implemented, a better method of travel would be sought due to the hazards of building bridges over this terrain. However, both air and train travel would lead to an expansion of Hyrule's borders as new lands were discovered. Other races/cultures would be discovered as well. These cultures would mingle with the residents of Hyrule, and naturally the population would increase. If no natural disaster as devastating as the flood or man-made disaster as population-wiping as the Imprisoning war occurs between
TP and this Future Zelda, the we'd finally get to see a populated Hyrule that would require ascension of new land and be interested in diplomatic relations with other lands beyond their borders. We'd see a larger Castle Town, which would be more of a city than a town. (Karariko Village -> Karariko Town, etc.) There would be a larger demand for food, as well, and it's natural that there would be an expansion of enterprises such as Lon Lon Ranch and the livestock/agriculture villages such as Ordon; So it natural to assume that parts of Hyrule Field would become subject to farmland. One could theorize that Kokiri Forest would be swept aside for room, but the spiritual culture of Hyrule is too strong to disregard things such as the forest, especially knowing that particular forest's history. In short, the use of transportation technological advancement in an unhindered Hyrule would open the world up to the player in a way it has never been before, and in a way that the Zelda developers have been hinting at for years.
With technological advancement in transportation is natural to assume that advancement would follow in other niches of technology. However, I feel this would soil Hyrule with too much of our own world. It's better to keep things at the steam and burning oil level, and avoid electricity. But, in order to push how far Zelda could advance before it's no longer Zelda, it's important to analysis something that is often overlooked in the series: magic. In the newer games it is namely Zelda and Ganondorf who display magic, and Link's use of magic is fairly limited to what it was in previous games and what it could be in future games. Link's sword used to shoot lightning bolts (magic bolts) at his enemies. It'd be great to bring this element back, especially with Wii graphics. Besides increasing the use of those chosen by the Gods, I see magic leaking more into the citizens of Hyrule. Several other races have exhibited different forms of magic (Geurdos, Sheikah, etc.), and, if my theory about mix breeding and increase of population were true, one would expect magic to become more every day. Magic would become the driving force behind technological advance rather than science. That being said, I still expect more metal and gunpowder to make an appearance in future games. Gunpowder isn't as exact as bullets, so the art of swordsmanship would prevail, but it would add more variety to weapon choice for Link, and other characters. Continuing with weapons/items. I see the Hero's bow fading away in a steampunked Future Zelda, and becoming a crossbow, which seems very likely after Crossbow Training. Also, with the combination of increased amounts of metal items and metal structures and magic, I see the two interacting more, especially if we continue on this idea of Link using his lightening-esque powers more (possibly some sort of metal rod in-addition to his sword, as a tool for channeling the magic, which would work nicely with puzzles). A return of the magnetic boots, as they were seen in
TP, would accent this lightening/metal idea nicely, both logically and visually. Replacing Link’s shield to give it a more rustic-bronze look will also add a nice visual element, and, since it is metal, will also interact with magic, similar to how the mirror shield acted in Ocarina.
Turning away from technological theories to plot theories, I would hope such a Future Zelda would focus a story that brought together the Twili (only reference), Sheikah (return of), Dark Link, and Vaati. All of which share vague, but strong connections, that if all drawn into one story, would fill in a lot of canonical blanks for fans. It'd also provide us with two popular villains that are not Ganondorf for a change (it'd be nice to see Dark Link play a larger second-in-command role alongside Vaati).
Below is just a quick mock up of boxart I had in mind for such a Future Zelda. It's very rough, and there's a lot of things I'd change about it, but I'm way too busy wit finals to work on it longer. The background had a train, but I scan with too many layers of transparent paper. Once again, not much time, but you'll notice I gave Sheik that lightening rod I was talking about.
