Legion should have ended with a much bigger loss than we got. Sargeras should have succeded in bringing about a near apocalypse for Azeroth, bring desperation to the world and making our win very bitter sweet.
Would have made for an interesting follow-up expansion. BFA would have been an expansion filled with desperate events, people trying their best to hold on and with a world dying all around us. Saving Azeroth would have been much more urgent and a constant fight to keep back the flow of destruction on Azeroth. Would also have given space to a patch, that simply was about making the best of it, an "End of the world party!" held by Goblins.