The organic nature of what the game is was lost somewhere along the way. Probably after Wrath. I'd say BC, but both pulled on monstrous lore aspects. Cataclysm had that opportunity, but people didn't give a fuck about the black drake when he was covered as far back as BWL, with no additional lore between two expansions.
A few people said it, but the "right" way would be to branch WoW Classic.
Run WoW Classic as Vanilla, and after Phase 6, announce the first expansion to WoW Classic... The Shores of Northrend.
You would run Classic as a complete fork of WoW, reusing the massive assets produced for the current game, but keeping the classic design. I'd skip over Outland for now, since Northrend would not be easier to redesign than Outland, but it would be a very good test bed to how to do such a Classic line.
Guiding principle: keep every quirk and weird from vanilla, remove only the worst part that really, really don't work, integrate only the parts that fit.
No new races, since you skip Outland. No Jewelcraft (you might salvage it, but it's an odd). No DAILIES (you can keep repeatable quests from vanilla, but dailies? GTFO). No heroic mode for dungeons; if you want heroic modes, you make "max+" dungeons tuned slightly above. No 10/25 man raids, you'll keep 20/40. And, of course, the WoW devs are happy that flying does not exists.
Northrend is also a good place to go, since you're fresh out of counter-attacking, destroying Naxxaramas and blunting the LK's assault. Instead of sleeping for a couple year until someone randomly opens up the Dark Portal and you feel the need to fuck up Illidan instead of letting him go against the Legion, you push back and counterattack. So straight Nax->Northrend makes sense.
The main challenges: cleaning some of the storyline. Redoing the stories since phasing is shit and needs to go. And if the quest writers complain, remind them that "limitations breed creativity". You need to redo the whole phasing, which means mainly that:
- You need to redo the Wrathgate story (can't phase Wrathgate, can't invade phased Undercity, and Saurfang Jr is still hidden away on Nagrand anyway)
- You need to redo the entire dual factions of Sholazar (no phasing, no dailies)
- You need to redo the whole Icecrown (it's absolutely insane to navigate without flying, plus massive phasing to invade)
- You need to redo the Dalaran/Blue Dragonflight issue (the concept of a neutral capital is one that doesn't fit Classic)
Things you need to keep:
- +% crit/haste, etc. The main issue that comes from the handful of items that have only/more than half +%. The +% value grow over levels, but if the rest of the item doesn't grow, it will become lesser over time.
- Skills. Keep that.
- Horde Paladins (Taurens, of course) and Alliance Shaman (Dwarves, of course)
- Argent Dawn. What's this Argent Crusade shit?
- Alchemy Specialization, probably add another (Mining Spec: more elements or more gems?)
- Faction Tabards. The best invention of LK, I think.
I'd do the inverse of the planned level squish: Make the expansion 60-75 instead of 60-70. Quest items at lvl 60 in Howling Fjord have the same ilvl/items as greens in Winterspring, and stretch more slowly items. Make naked stat growth slightly slower; the idea would be that 60-75 is equivalent to leveling from 30-60. Since you don't have spell scaling and talent trees, you still yield lots of new ability per 2-3 levels.
That's a rough outline. You need probably a replacement for some stuff (Coliseum - without the dailies, it doesn't make sense). I'd also use this as a testbed for cut-n-paste creation of original content from existing maps. Glue together a handful of titan-themed places and see if you can add a dungeon in Sholazar for example (with the Oracle/Frenzyheart conflict reused as an intro for the dungeon): The Titan Archive.
You can sell that to the beancounters by tying it to the current expansion: if you don't purchase the current expansion, you don't have access to the Classic expansion. Probably would be a hard sell as a separate expansion since it's a copy and paste.
The biggest problem: as you said, if that shit happened, you would have 80% of the players playing Classic rather than Modern.