Stonetalon Mountains has the most unused potential
Map explored with [https://wowforevermap.com/](https://wowforevermap.com/)
Stonetalon Mountains easily has the most unused space, almost making up half the zone with only some texture changes to the western mountains compared to Classic aswell as a new mine above Cragpool Lake (Cyan) that remains unfinished. We did see Skywatcher Plateau (Green) but while it has a path into Stonetalon I think its really its own mini zone and technically part of Mulgore.
If we take inspiration from Warcraft 3 then maybe the Oracle Cave as a dungeon to the West and maybe some ruined Human settlement in the East? Grom did fight a lot of them during the campaign and it seems odd there are no remnants of that in WoW away from the Barren Coast.