I'm only just replying to this thread because I've spent most of the day downloading and checking the 32.5GB that make up the Nether.
This image shows all the chunks that have been generated around spawn, with 0,0 being roughly in the middle. Anything that's black are chunks that have not yet generated, and if explored in 1.16, would generate with the 1.16 terrain. Anywhere that's grey are areas where the entire region file hasn't been created yet, so every chunk in that region would be up for fresh generation in 1.16. The orange areas are chunks which have had a player in them for less than 30 minutes total, since whichever update added the counter (I think the one before ocean monuments were added, so 1.7 or 1.8 maybe?). Similarly, red ones are region files where every chunk in the file has been inhabited for less than 30 minutes. The gridlines are every 512 blocks. As you can see, the only areas of the Nether which have been in heavy use make up a circle around 2000 blocks in radius.
This second image shows the current Dynmap view of the Nether. It's not fully up to date, so not every chunk is shown on it, but it does highlight all the protected regions in the area around spawn. The scale isn't perfect, but it's *roughly* the same scale in both images. This shows, again, things are mostly contained to within the immediate area surrounding spawn.
I think if we were to reset anything, I'd personally be strongly in favour of keeping the 2000(ish) radius circle around spawn, as well as any villages currently protected.
However, I also don't think there is a super vital need to regenerate the entire Nether. The way it was dealt with in the End was by Mojang adding in the portals to take you further out. It would be super easy for me to add 4/8/16/32/howevermany portals that come out in a ring at around 10,000 radius, which would be plenty far enough for new terrain to be generated. What's more, with the new biome areas we'd be opening in 1.16 mentioned in the "New World" thread, each of those locations would be sufficiently far out in the Nether that it would be guaranteed new terrain. The End has 20 portals which generate, and each of them end up miles apart from each other when further, out. In the Overworld, we were going to limit how many we used so we had room for expansion when new biomes came out. I can't forsee Mojang changing Nether generation massively for a few years after this update, so theoretically there's nothing stopping us adding as many portals as we want taking you further out. We could start out wit 16, and if they all get used up, we could add another subdivision between them.
Yes, it's easy to move structures. It would be even easier to move entire region files, which would avoid a LOT of tedious work involved in making sure everything gets copied over to the right place. If we did decide to regain the entire Nether, there are ways to copy things over.
That being said, I'm not personally convinced it's all that necessary. Due to the 1:8 ratio thing, the Nether isn't like the Overworld. Not that much of it has been explored. Now we have elytra it doesn't take an awful long time to travel out to bits that haven't been explored. We could make proper ways to the roof for easy transport, or we could do the portals as mentioned earlier.
TL;DR: The Nether isn't as big as you might think, but it's also very much not empty. It's mostly all contained within a smallish circle around 0,0, with a few villages around the place. It would be possible to move things over, the exact method would dictate how difficult it would be though. My personal view would be to use a solution similar to that in the End, where we add portals to take people further out, which combined with the overworld portals, would provide plenty of room in the Nether for people to explore for the foreseeable future.
