Within the context of Classic, the method most frequently proposed is to re-tune bosses to make them harder. Rather than copying the numbers, re-create the relative difficulty. Kind of like, when you first start working out, you use small weights. When you get stronger, you lift bigger weights. The WoW community is more knowledgeable, more experienced, has better hardware, etc. We're "stronger." So give us tougher bosses. There are a lot of forms that could take. Bosses were constantly re-tuned during vanilla anyway, so if Ragnaros does 10% more damage or something, is that really a problem? I don't think it is. What about resistance gear? back in the day, collecting resist gear was a thing people did. But the numbers and strategies are so well known today that only the tank really needs it anymore. Bosses could be re-tuned with more resistible elemental damage so that's important for entire raids to gear appropriately. That would be entirely keeping with the spirit of vanilla even if the actually numbers sitting in a database somewhere were different than they were ~13 years ago. If you want to go a little farther, a few people have proposed adding new abilities to old bosses, adjusting their timing, etc.
If you’re eager to claim your character name in WoW Classic, take note: we’ll be opening character creation on Tuesday, August 13.** Players with an active subscription or game time on their account will be able to create up to three characters per World of Warcraft account. We’ll have more information on realm names closer to launch, but rest assured—you’ll have plenty of time to figure out your plans for realm domination!
At some point, Zygor was destroyed. Accounts differed as to what the cause of this was and when it happened. According to one account, the planet was destroyed in the war against the Xaranti. (PROSE: The Bodysnatchers) Another account given by the Zygon Broton in the 20th century said his world was destroyed in a "recent catastrophe". (TV: Terror of the Zygons) Another account, given by Elizabeth I who had been posing as a Zygon in 1562, said that the Zygons lost their world when it burned in the first days of the Time War. (TV: The Day of the Doctor)
Occasionally it's also been proposed to re-randomize all the gear. We didn't have best in slot lists back then. People weren't starting out as a fresh level 1 character with a complete list of which dungeons to run at 60 in order to gear as quickly and efficiently as possible. Now, all that data is available. If gear were re-randomized then all of those lists would become useless and thing would be close to how they were. Which is a more important part of the "vanilla experience?" the fact that item X from boss Y has exactly Z stats on it, or the voyage of discovery? Personally I think that voyage is way more important. Do you remember how many points of which stat on the ring or whatever that Darkmaster Gandling drops? I don't. Would it kill your experience if the BiS for every class were in a different dungeon so people had to actually play to figure it out rather than simply consult a spreadsheet? I don't think it would. For that matter, it probably wouldn't be a bad idea to handle it intelligently so that the better gear is in later dungeons. I remember there because some weird cases where level 40ish gear was better than anything that dropped in level 60 dungeons.