The true mistake WoW did in terms of writing was to kill of the Lich King.
WotLK should have ended with the Lich King victorious, alliance and horde scrambling to make an escape out of Northrend, dreading when the next blow would fall.
It would also have helped limit the power creep the series was dealing with, if you establish the Lich King as the big looming threat in the background, and sometimes foreground when he invades a region of the world, you can't make villains that are significantly stronger than him.

Shadowlands should have ended with Arthas and Ner'zhul usurping the power of the Jailor, breaking open the afterlife and ascending back to the surface to reconquer Northrend as the Lich King reborn, now with the ability to bring forth undead straight from the Maw

Would have actually been a fitting consequence for Sylvanas: make her directly responsible for the Lich King being reborn