Just re-tell the story from the start in a new game in a better way (or try to) which means a new game engine and better graphics and better mechanics and delete all old players achievements and items and transmogs and keep them only as feats of strength (because oh boy some old players WILL cry but you have to rip the band-aid to improve) and maybe only keep it as a "classic server of Shadowlands".
Besides: that's exactly what WoW did to Warcraft: it re-told a story; why not do it again?