If I was Blizzard, I would not progress classic (after Naxx) following standard time line, and standard expansions.
Instead I would progress doing things completely different:
max level would always remain 60 forever
there would be no nerfing of any content or speeding up exp in any way (every single new coming player would have to go through everything, any player before had to, and spend the same amount of time to do it)
I would introduce new lands (that came with expansions) slowly, but would be rebalancing all that for level 60, as post naxx content (so for example TBC lands, would be very hard, and require players to have AQ40 minimum, and Naxx recommended gear. There would be no jumping to new lands as soon as you level up.)
When new post-TBC lands open they would also be balanced for level 60, but for a gear from TBC raids.
level 60 PvP would be separated in brackets (same like pre 60 PvP is) depending on what point of progression in PvE player has reached (it would get tracked through some achievements, raid clearing and quest line completition, and NOT by gear you are currently wearing), so players who are currently in point X of content, would fight against players who are in same part of progression
each bracket of level 60 PvP would grant you different tokens, that would allow you to purchase PvP gear that fits that content level
PvP ranking would never get removed. Gear would be purchesable through effort (tokens gained) and would not be restricted by rank, but RANKING would always remain to separate players who show skill and effort, so they can show off their titles and rank based cosmetic rewards (special mounts etc).
Yes, in time that would create a HUGE gap between new coming players and veterans. But this is as it should be, because each player should have to (it's not a minus, its a PLUS) pass all the months / years of game play at appropriate content level to eventually reach current endgame.
Time gap could be shortened anyway by having veteran friends boosting you a bit, if you chose that.