I’m about to start the Emerald Dream questline with my druid.
Was wondering if this is account wide.
Will I have to start it over with other characters if I finish it with my druid?
Will the druid forms will be shared with my other druids?
The questline itself isn’t account wide but unlokcing all the repetitive stuff and access to Emerald Dream is account wide. So if you want to go through the story, unlock armor second color for heritage set for Night elf(if your durid is not a Night Elf) or get transmogs for other armor type you need to complete that questline again on another character. Otherwise there is no need to do it again.
Barbershop customization, even unlockable is account wide, this includes druid forms, warlock demons and various skin colors or something unlocked via questlines.