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.
1 All expansion guides are written in the Battle for Azeroth beta in order to have them ready on launch day. It's important to note that betas can be buggy and unstable and may delay and even halt our progress on the guides. Furthermore, the time between the start of beta and the announced release of Battle for Azeroth is much shorter (by about 2-3 months) than past betas. As a result, our primary focus is to have the Leveling guides updated by the launch, and only if those are complete and time remains will we work on updating the remaining guides. All guides not finished in beta will be updated post launch. You can read more about this here.
The system is automated, meaning that once slots are available, beta testers will be chosen, without any beta key to speak of. Kaivax then goes to stress the importance of your account security, stating that, “any third-party e-mail messages, websites, or in-game whispers that offer beta keys or access to the WoW Classic Beta should be considered illegitimate and ignored or reported.”