Well, each expansion had a "poster class", so to speak - mainly the class/job the Warrior of Light (the stand in for our character) took on in the cinematic trailer.
So that would be Warrior for ARR, Dragoon for HW, Samurai for SB and DRK for ShB.
I'd say you could argue a bit about ARR, considering how it had a bit more focus on an adventurer-party, with a warrior, whitemage, blackmage and bard.
Given the far eastern theme of Stormblood and Lyse being one, monk could also be considered a featured job for that expansion.

Its a running joke by now that the "poster-job" is the one that gets messed up the most in its respective expanion...