What are the species or subraces, whenever they are from the Alliance or the Horde, or independant that should work together in the story or who have already worked together but not to the full extent that they should or whose work haven't reach their true potential yet, whenever it's in the construction, culture, military, magical, transport, technological and other kind of uses ?
I definivetely think that the Forsaken and Goblins should be shown working together more, with the Goblins being the only race of the Horde capable of enhancing or completing the Forsaken's use of biological or chemical weapons or use of dark magics with their very advanced, destructive and often very unpredictable technology and alchemy.
We should also see the Draenei and their Lightforged kin cooperate more often with the Night Elves, with their crystaline and more respectful of nature technology and magic being great assistance to the Night Elves' stealth, mastery of nature magic and connection with strong wild creatures such as Ancients, Fae Dragons, Chimaeras, Mountain Giants, Wildkins, Furbolgs (with the Stillspine tribe being a common friend to the Draenei and Night Elves), etc...
It would also be cool to see the Draenei help the new Night Elves mages with their experience and knowledge of magic, and to see some discussions and discoveries about the ties between Elune and the Naaru.
No need to talk about the Blood Elves and the Nightborne, with both elven species having a great mastery and knowledge of arcane, though working together could allow each race to learn new techniques and fields of magic they weren't aware of before.
The Gnomes and Draenei could do some very awesome work together, with the Gnomes' creativity and quircks being quite balanced by the Draenei's wisdow, and seeing more of the Draenei and Lightforged' magic and tech could lead to some new directions for the Gnomes mages and tinkers, with some potential real progress in the use of the Light, while interacting with the Gnomes could make the Draenei and Lightforge discover some uses of their tech and magic that they hadn't thought about before and inspire them to be more creative.