For me, must-have are the following:
Tengu and the Dominion of Winds. Maybe we won’t have the Tengu as a playable race (though I hope we will) but not having them as an important aspect of this Expansion (or the living story that follows it) would be the wrong decision.
The history of and relations/hostilities amongst the different underwater races (Krait, Naga, Karka, Quaggans and Largos) and between those races and the deep sea dragon.
Cantha’s relation to spirits and ancestors as seen for example in the Envoys, the history of the Ritualist profession and the existance of Tahnnakai Temple.
The relation between the Jade Wind and the Affliction on the one hand and dragon magic / dragon corruption on the other hand.
The origins of Kuunavang, her relation to the Elder Dragons and her possible relation to Glint.
The history of purification in Cantha after GW1. Starting with purifying Cantha from the affliction and then contuining with purifying it from other races (such as Naga and Tengu), eventually going so far as to (try to) purify the Kurzicks and Luxons from their own cultural heritage.
Bureaucracy, corruption, tradition, inequality and street gangs. To me these elements are quintessentially Canthan.
The effect of the rising and defeat of the Tyrian-based Elder Dragons on the Canthan continent.
Elements that might be interesting if they are touched on but I can do without or that might not even have a place in the expansion:
The Dredge, their tunnels from the shiverpeaks to cantha and Primordus.
Possible links between the Wardens and the Sylvari
The history of humanity on Tyria. There is some conflicting lore on whether humans were first brought to Tyria in Orr or whether humanity came from an area to the south of Cantha.
The battle isles.
The tentative suggestion that margonites might have been luxons that sailed away from Cantha before the Jade Sea was turned to jade.
That is all I could think of at the moment, though there is probably a lot more. So what about you guys. What do you want to see in the next expansion?