Perhaps I should have said "the cosmic trilogies", because that would be more accurate. Not each expansion is cosmic-focused. But the overarching story it is a part of has a cosmic theme.
TBC -> WoD -> Legion is Disorder (Fel magic). The ultimate threat is the Burning Legion, commanded initially by Archimonde and Kil'jaeden but finally by Sargeras.
WotLK -> BfA -> Shadowlands is Death (Necromantic magic). The ultimate threat is the Scourge, commanded by the Lich King but finally by the Jailer.
Cataclysm -> Dragonflight -> 11.0 is Shadow (Void magic). The ultimate threat is the Hour of Twilight prophecy and its adherents such at the Twlight's Hammer and (indirectly) the Primalists, commanded by the Old Gods but finally by whomever ends up being the bid bad. Xal'atath?
The first expansion in each trilogy just picked up where that theme left off in Warcraft 3, and went big with it. Then once they had done those, they picked each thread up again after MoP, with a set of two expansions for each cosmic force.