What I mean by regression is reverting some game play feature changes they have made in the past, in light of newer systems.
Example:
Topic: In Vanilla and BC we had beautiful long attunement quest lines, that spanned patches. This would generally be a reputation requirement and quest structure that spanned the world, instances and raids.
Cons: During that phase of the games life cycle, it was harder to accommodate these things because it was very convoluted, leading many to having no clue of where to even begin them.
Pros: With the level of communication we have now, the dungeon journal, and the matchmaking system... this would fill the time between patches and have none of the side effects (unless you re-rolled a character, but who can play wow anymore on just one character? Certainly, it is nothing to meet the requirements on multiple characters between patches). When the raid is launched, you spend that patch raiding and working on your next attunement for the upcoming raid. When that raid releases, then you can do away with the older attunement if you wish. But attunements were driven content.
This would deliver a more RPG feel to the game, and provide reasons to log on other than raiding. If utilized properly, it would be much more content that some patches we get now, but would fit back nicely into the system.
Topic: Why do we not have World Quests in every zone in Azeroth since this is the "faction war" expansion (even though it is most certainly fluff leading up to a VOID expansion?
Cons: Blizzard doesn't like sending people to old zones because they are sort of outdated with the end game at the moment. Then again, they have no issue with incentivizing leveling of allied races through that very same outdated content.
Pros: Would actually get people out in the world and populate it again. It also provides a great deal more story content that what is happening on Zuldazar and Kul'Tiras. This is after all a fight to save the world... are we to expect that all the other zones are just filled with NPC's milking the cows and producing next season's offspring? Certainly they would have interesting perspectives on the faction conflict. It also opens up potentially more balanced Warmode (world pvp) because it spreads the population out a bit. When everyone is tossed into a closet and told to fight it out, of course we will end up with warbands like we did.
Topic: Instances. I would like to see the return of Burning Crusade instances being tied to their respected reputations. Back in BC, if you wanted to enter Hellfire Ramparts on heroic, you need to get to revered with that faction first. It wasn't just a race to WQs back then.
Cons: It would slow down early expansion progression a bit, but that's not really an issue when they already hold the raids back a bit.
Pros: You wouldn't have players sitting on the cusp of raid gear before they even enter the raid. BFA was extremely weird in that you did not have to even enter a heroic after hitting 120 if you had a group ready to go. We went straight into mythic instances, getting gear only slightly lower than the raid; a month before it even came out. It wouldn't slow down the player base dramatically. But it felt good running instances to gain more rep, as you only got to revered with normal. You had to clear heroic a few times to get exalted. Then you could go purchase that item, that would give you something to go raiding with.
Topic: Why not bring back the old pvp ranking system? Yes, the titles were given to RBG players (you can buy High Warlord now with ease). But I found the grind back then to be more fun than the grind nowadays with the "prestige ranking" system. Random BGs were incentivized back then, and not just with gear.
Cons: You would have to find new titles for the RBG system.
Pros: Gives new life to the random PvP scene, because people will farm for those titles. Less constrictive then RBGs.
Just a few thoughts. Can you think of any you would like to see? Note: It should be things that could benefit the game, but not utterly call to destroy everything new if it were brought back.