Gnomeregan and Gilneas should have been restored and reclaimed a long time ago, the fact that Blizzard keep things stagnating is really outdaded and frustrating. Show more of gnomish and worgen cultures, and give and develop more characters for these races.
Also show the Azuremyst Isles truly become a home for the Draenei, see them building a true city and more settlements here with Draenei architecture, such as the ones we saw on AU Draenor, also show more of the interactions and issues between the pure Draenei, the Broken, Lightforged and purified Manari Eredars. Also develop the Draenei culture and tech and magic more.
A bit more of exotism would have been welcome a long time ago in the Alliance, the allied races were a missed opportunity for this. I have always thought that the Arakkoas, whenever they are the ones from Outland or from AU Draenor, or Botanis, or one of the Sethrak tribes should have joined the Alliance.
For the big strong allied race, I think that the Vrykuls of Stormheim, at least one clan, could have filled the role of the Kul Tiras humans allied race, gameplay wise, because in lore there was nothing to say or foreshadow Kul Tirans as bigger and stronger than other humans during Warcraft II, Warcraft III, WOW and the books until BFA.
And as others and myself have saide before: stop making the Night Elves a punching-ball and allow some moral ambiguity and grayness in the story without having the Alliance doing things outright villainous à la Garrosh, some Bloodraven character for example, to be the leader of Stormwind warlocks could have been a great addition to the story.