WoD definitely did not introduce Gul'dan.
Both Gul'dan and Sylvanas were already established characters by the time they set things in motion.
While the lead-in to Legion was somewhat written on the wall, during BfA that conclusion was far from obvious. I mean just as many people claimed Azshara was being set up - you know as the antagonist in the upcoming void (or
underwater /s) expansion.
The problem isn't that Iridikron isn't being set up as a future antagonist and agent of the void (he clearly is) - but that doesn't mean the next expansion is definitely a void expansion.
I mean he could be Garrosh in MoP setting up WoD which would then set up Legion through Gul'dan.