Let us create a list of all the abandoned plotholes Blizzard has dropped. I'll start with what I can think of so far:
-Humans choosing to become Worgens in order to not be risen as undead. This went in well with the dislike towards Worgen and viewing them as evil creatures (sadly this got dropped beyond Cataclysm for some reason)
-the red pox, Orcs and Broken Draenei got mutated and can not hold power from the light anymore
-undead grain, a plothole that the Legion could've used again, but was never ever brought up
-the Fel creating necromancy; basically the Fel was a way into getting necromancy, however this has probably been retconned with Shadowlands
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If you have any other forgotten plotholes, please let me know.