>>96150287
Reset the timeline to start of Classic, but include various regions that were only elaborated on later: Gilneas is fighting a civil war over Genn's Wall and dealing with the first outbreaks of the Curse, Arthas up in Northrend is consolidating power to launch a new invasion of the world, the Blood Elves are casting about for allies, etc. Fix all the Blizztardation and lore contradictions, hire some writers who actually care about the setting and some editors who actually care about quality. Many of the racial/minor plot hooks from Classic were actually pretty good, so keep those and use them to give GMs some easy ways to make adventures.

Change the mouthfeel of the world and art from what it became, lean more on how empty and devastated the world of Azeroth is. Lordaeron and Alterac have been obliterated, completely depopulated apart from some undead squatting on the ruins. Stormwind is in ruins, the city itself a massive refugee camp still being rebuilt from the devestation of the First War. Stromgarde has collapsed into banditry and petty warlords. Kul Tiras and Gilneas have shut themselves off from the world and the latter is about to consume itself. In Kalimdor, the Night Elves are broken race trying to build a new home. The orcs have found a home, but their numbers have dwindled from a mighty horde to a few tens of thousands, with only one tribe of tauren and trolls each as allies. This is a new Dark Age, and while there's hope and light and the worst of the Apocalypse is over (for now), things are still going to get worse before they get better.

As for systems, obviously a in-depth race/class system with plenty of gameplay implications is important. Some sort of adventuring band system like >>96151575 suggested. Move the plot forwards slowly with adventure books and new editions, taking vanilla the expansions as a rough guide (but ignoring any and all shitty Blizz writing) up through perhaps MoP. After that, go in a new direction