As everyone knows current leveling timeline is complete mess. Most players start in Cataclysm then go back in times of TBC and WoTLK. And only then they come back in Cataclysm timeline and things gets normal. Death Knights and Pandarens have even more messed up timeline - they jump from expansions timelines back and forward. It mostly caused by Cataclysm revamp and i believe next revamp can completely fix current mess. There idea:
New Revamp of Kalimdor and Eastern Kingdoms:
1 - It's not secret what in last chapter of Legion we deal with Sargeras and his Burning Legion. And its not secret what Ashara and her Naga is next threat looming on horizon. New Old Gods invasion is good reason to complete revamp main two continents and update it's world to current timeline. And i'm talking about DEEP and CONTRAST revamp of current zones. Turn green forest of Feralas into dark and grim haunted forest. Make Plague Lands cleansed. The more changes the better. Blizzard can even change current capitals balance - destroy Stormwind/Darnasus/Undercity/Thunderbluff and rebuild Exodar/Silvermoon.
2 - After revamp Blizzard locks access to Outland/Northrend/Pandaria/Draenor/Broken Isles. Lore reasons why we can't go there can be different. For example: after Naga's (New Threat) attack Alliance and Horde lose it's ships/portals etc.
3 - Old expansions zones and access to them Blizzard put in new shrines in Caverns of Time. Total 5 new entrances in CoT - Outland, Northrend, Pandaria, Draenor, Broken Isles. New Kalimdor and EK zones added in Cataclysm can be accessed through CoT NPC in those zones (phasing).
4 - Finally put Exodar/Silvermoon in Kalimdor/EK continent world and allow flying there (Current Exodar/Silvermoon technically located in Outland). Also new starter quests for draeneis and blood elves!
5 - No need to change start for worgens/goblins/death knights/pandarens/demon hunters. Just call their first quests and zone like some "Long time ago..." session. So after they finish their starter zone they continue leveling in current (new 8.0) timeline.
6 - New Kalimdor and EK oriented for 1-110 levels. Rest levels people get in new expansion zones.
PS Another more radical way to change timeline mess is completely wipe whole world and remake new. But ofcourse Blizzard wouldn't make it.