“Best” is pushing it, especially with these writers, but… “Tolerable”? “Unobjectionable”?
Right now, Zovaal is staggeringly banal, memorable only for his poor execution and utter void of characterization or establishment. This goes for entirely too much of SL, but in his specific case…
They absolutely needed to foreshadow and establish this guy far, far better. And they had a chance to at least get something done.
Throughout BfA, make it clear that Sylvanas has some super scary extradimensional patron. Throw in some references to Helya and Sylvanas’ deal with her, strongly hinting that there was some third party involved that’s beyond both of them. Have her use more of her Maw powers early on. Maybe have some of the Mawsworn show up early, making it clear these aren’t her standard Valkyr. Make Baine’s bonds be forged from the Maw. Make it clear that, beneath the war, there is something else going on.
Then, when it comes to destroying the Crown of Domination… give credit to the Jailer. Do not make it another demented, “OMG, isn’t gothy waifu the COOLEST!?! *_*” moment - have Bolvar beat the everloving death out of Sylvanas, have her get off one lucky shot that cracks the crown and let’s the Jailer throw his weight around (and make it clear that this is not coming from her), and then have him obliterate Icecrown, Sargeras v Silithus style.
Even if he was a totally blank non-entity going in, letting the Jailer show just how insanely powerful he is/can be would have garnered infinitely more respect than… whatever the hell they tried to do with him before hand. A grand enough display of power, even from a total anon, will make any engagement mean “suit and tie” levels of business real fast.
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Do that, and at least the Jailer could have been a “good enough” villain. As it stands, they just plunged into trying to make a “Thanos” without putting any of the “entire MCU” work in. And the results are clear. People want the Jailer dead just to get him off stage and move past this story, not because he’s any kind of love-to-hate villain anyone really wants to watch get beaten.