In and of itself the idea of Illyana becoming the X-Men's big bad is not completely without merit. The point of her character is that she struggles with a dark side and that the only reason she isn't the worst of all villains is that her willpower and desire to be good are that strong. There is room for a story where her strength finally fails like Frodo's does when he finally reaches Mount Doom and he gives into the temptation of the ring rather than destroy it.
The biggest problem is that the best times for such a story have long passed. The original Inferno was the best time to do it, when she was at her lowest believing her brother and Doug were dead, facing a never-ending rebellion in Limbo, had practically no control over her powers, and was beginning to kill her enemies and forgetting everything she learned during her original captivity in Limbo. But Inferno depicted her as heroic throughout no matter how much her physical appearance changed, and it ended with her making a heroic sacrifice.
The second-best time would have been immediately after her 'there are no snowflakes in hell' speech, as that was when she was at her most unheroic and had given up on even trying to be good. Ever since then, she has been on an arc of becoming more heroic and more mentally stable and gaining control over her powers.
During the Bendis era, Illyana started trying to be good again and trained with Doctor Strange to finally control her power.
Since then, she has seen her brother killed in battle, witnessed a slow-moving genocide of her people with the terrigen mists, been forced to mercy-kill her beloved student Sapna, been betrayed by one of her best friends and made a prisoner of the US government, and witnessed her soulmate Kitty die at Shaw's hands, all without turning in. Even when her mutant side was destroyed and she became a feral beast for a time, she didn't kill Juggernaught and threatened no one outside of the single room where the event occurred and presumably got herself under control again.
And now, we had a story where she transformed and got a more heroic costume to show her more heroic nature, and a story where when given true godlike power she is not tempted at all and uses that power only to heal and restore before giving it up. Plus she has given up control of Limbo entirely in the first story, and without Limbo she will not make anywhere near as good a villain as she would have in 1988 or 2012.
So unlike Frodo, who was steadily getting worse until he finally succumbed, Illyana has been on the complete opposite path and is not in a place where it is realistic at all for her to give in to her dark side. Any story where she becomes the villain has to be triggered by an event that is even more traumatic than the death of Colossus in Battle of the Atom, the death of Kitty in Marauders, or the death of Sapna. A betrayal by someone close to her can't be the trigger either since that was already covered by Karma's betrayal in New Mutants Dead Souls. Without such a traumatic trigger, the story would fail from page 1 and be nothing more than editorially mandate crap that has nothing to do with character development or anything resembling good writing. It would be like Superman turning evil in Injustice because he got up on the wrong side of the bed and not because the Joker tricked him into accidentally killing Lois.
Assuming such an appropriately traumatizing trigger can be found, the first thing they'd need to do just to establish her as an effective villain is to have her reconquer Limbo from Maddie, undoing everything The Labors of Magik accomplished. This is another factor that an evil Magik story cannot go without. Conquering Limbo from its current ruler would go a long way towards establishing her power on the individual level, and again, it is only as the commander of the demon armies of Limbo that this kind of story can work at all.
And even if they handle it perfectly, with a perfect setup that explains exactly why she'd go evil now and the perfect use of her as a threatening but tragic villain, it would likely still not be worth undoing all the character development of the last 11 years, wasting one of their best heroes, and getting rid of what looks like a good thing they've got going with Maddie right now, just for a 'Magik goes bad' story. Even a perfect story would seem out of place, out of character, and like an editor has an ax to grind rather than a natural story or character development, since it would go against everything that's been done with the character since 2013.
Basically, in theory such a story could have worked many years ago. In practice, it can't work anymore and doing it now would be a huge mistake.