Doctor Strange in the Multiverse of Madness could be considered, in a word, mind-bending. Its chain of events saw Strange thrust into a battle against ally turned enemy, Wanda Maximoff, who wanted nothing more than her children to be real. Their chase took them across the Multiverse, and Strange had to break his own rules to save reality and stop Wanda. But some audiences felt that this version of Wanda was a contradiction to her growth in Wandavision, to the point that one change could've made everything make much more sense.
Ever since Loki, the concept of variants has existed as various versions of the same being from different universes. This would mean that a version of Wanda could've been much stronger or weaker, like the one Wanda dreamwalked into to decimate The Illuminati on Earth-838. Therefore, Multiverse of Madness could've introduced a variant of Wanda that had never dropped the Hex in Westview to serve as the main villain instead of Earth-616's Wanda.
A choice like this would've also maintained Wanda's character arc from WandaVision, where she showed remorse for her Hex and let everyone go so they could return to their families while she lost her own. That one action showed that Wanda's evolution into The Scarlet Witch wasn't a curse but a liberation where she finally had control over how she felt and was ready to take her next steps as someone who wasn't afraid anymore. However, Multiverse of Madness negated this and instead focused more on how she was corrupted even though she saw the damage the Darkhold caused.
By having Wanda face off against an evil variant of herself, she would've had to literally face her dark past and fight her own urges to try and find the family she lost in Westview. Not only would her victory have profoundly affected her future, but it could also have left the door open for her to make future mistakes either way, such as erasing Mutants or even believing that she could still find her family without becoming as evil as the Wanda she fought. This internal conflict would've also been no different than Strange facing his evil self in Multiverse of Madness and how he did nearly the same thing by using the Darkhold to save America Chavez.
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While the idea of an evil variant of Wanda could've worked in Multiverse of Madness, there's also the crucial factor of Wanda's characterization and how trauma had affected her. She may have learned one lesson in Westview, but that likely never would've stopped her from making totally new ones when she read the Darkhold. Ultimately, she's only human and prone to mistakes that may seem innocent but could spiral into something far more dangerous. There's also faithfulness to the character from the comics, and that version of Wanda has made similar contradictory mistakes in her life. This is best shown through the various mental breakdowns she's suffered in her life. But no matter what, she's remained a hero and done penance for her mistakes.
Wanda Maximoff is a complex hero, and fighting an even more complex and corrupt version of herself would've brought this concept to the forefront. That said, it would've also detracted from her growth because it's in Wanda's nature to make mistakes and learn from them. Furthermore, her main struggles have always been internal, and to face them externally would detract from the internal strife she'd dealt with ever since she was a child.
To see Wanda's dark journey, Doctor Strange in Multiverse of Madness is now streaming on Disney+.