One thing alot of people fail to see when they think of a futuristic Zelda is that it would be the future in the world of Zelda (which is inevitable) and not our world or scifi like starwars (which funnily enough, has quite a few medieval sort of places which a good touch of the future)
That means their future would be dramatically different to ours. They have magic, they have gorons and zoras and the like which would dramatically change how their technology progresses compared to ours.
Take Majoras Mask and Windwaker for example, they clearly invented cameras before they used their gunpower technology to make the earliest guns.
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A quick example, say the game begins with a strangely dressed man and his mysteriously disguised companion (who is a clockwork robot, because its not far from current Zelda). They appear as if from nowhere/portal in the air/a time machine similar to the one in the movie of the same name.
They walk through castle town attracting alot of attention. They approach the gate to Hyrule Castle and the man asks the guards to see the king of Hyrule about something urgent that begins in a few days. When the guards ask the man why they should trust him and let him in, he shows them the back of his left hand which bares the crest of the triforce of wisdom and tells them he could trust them with his life.
Turns out this man is from the future, a future that has been corrupted over the centuries (or thousands of years if we go by the supposed time distance of
OoT and WW) by something that happened in the time he is visiting. This man is the current bearer of the triforce of wisdom and he was searching all his life for the one who bore the triforce of courage to help fight the evil in his own time. However when he found the lineage of all the heroes that bore the triforce of courage in the past, he found out the last one recorded was in the time that everything went downhill.
So realising that trying to find the current bearer was useless, he did the next best thing, he tried to find a way to get the last hero.
Using all the information and artifacts he could gather of the Temple of Time (destroyed mostly but the room beyond the door of time somehow stands untouched) he created a method to go back in time to find the courage bearer.
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So you begin the game as Link like usual, its a few days after the time traveller arrived in this time and already the events of the future are beginning. Eventually the traveller finds you and thats where the real adventure begins. You must travel between times with him, learning of what is to come in the future and preventing it from happening in the past, each time making a dramatic change in the future the next time you visit.
Take note this is not like Oracle of Ages or
OoT where the time travel is barely any difference, this is major time travel and your actions greatly affect the future. The future itself is incredibly different yet noticeably the same
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Because of the way time travels in Zelda, swords are still widely used and favoured over guns due to not much research being put into guns themselves. Crossbows are much more favoured over guns due to being faster and simpler to use.
Magic still exists in the future and many people try to keep it alive by mixing it in with technology. Hover technology does not exist but there are airborne vehicles (think of airships, zepplins, ect.) thanks to mainly magic.
Hyrule itself has changed alot but relics of the past are still there. Hyrule Castle Town still has some of the old buildings (though updated quite a bit) but they are in the poorer sections of the town. The richer and more modern buildings consist of ornate marble and crystal giving a sort of shiny metal and glass look common in some future designs but sticking with the ancient.
The gorons would have alot of machinery, they would be the kings of inventions, most of them looking steampunk.. They would have steamtrains, huge drill machines and more.
The Zora would be a bit more in touch with nature and magic but would be the masters of any watercraft.
The Gerudo would have a slightly cyberpunk sort of theme, alot of them pirating and such the air, roads, traintracks, waterways, ect.