Still super sad they didn't do this, I mean they even had magic setup as a bad thing in the primary show, imagine if Star didn't want to get rid of magic instead, due to all the magical creatures who live dependent upon it, but magic itself did something horrible to Marco like get his little sister or family hurt, or worse.
Like, the start of Evil Marco could've been his past friends and family were captured/injured so reoccurring villain could claim the wand, probably Toffee due to the prophetic dream of Marco dressing similar to him, and basically push Marco down a path of hating magic as well due to the corruption its existence creates.
His goal then becomes to destroy magic itself like the main show, except this time Star is defending it because the people she knows and cares for would be LOST in some cases. Marco steals the wand, works with Glossyarick who agrees with him on magic needing to be destroyed, and is FAR MORE HONEST this time around, and is actually legit trained by him. Let's say Marco left his jacket behind when he stole the wand because Star had decided to steal it or something to sleep with it, I don't know, but when Star notices Marco AND the Wand are missing, she immediately assumes that Marco has been kidnapped.
Marco then studies Eclipsa and Star's spells with Glossayrick, and steels his resolve to destroy magic, and lets have it turn out that Marco is EXTREMELY POWERFUL with magic, and have Glossayarick blatantly say that the strength of magic is an extension of the will of the user, the more willful and determined they become, the stronger the magic, since we SEE that in the show. Marco is an INCREDIBLY disciplined and DETERMINED guy. Like, MAJORLY. The dude left his friends and family behind to support Star due to current insecurity she was feeling and catching the feeling that SOMETHING was wrong. Plus he, with determination and not even KNOWING it wouldn't affect him, he spent 30 years trying to earn dimensional scissors FOR Star.
All because, at the time, she was his FRIEND.
This dudes magic ability should've been off the charts, even compared to Star, the dude doesn't hesitate to sacrifice his happiness for her sake, and trains like crazy.
So if he turned villain to PROTECT her AND his family from any more magic shenanigans and magic high commission playing with their lives, he'd actually probably be even more intimidating than Eclipsa AND Globgor COMBINED. If you don't think so, just picture him using magic to enhance his martial art ability, gaining more speed, strength, and even size. When the main character goes villain and single handedly smashes current history's most notorious figures, that sets a HUGE threat level, and would be extremely thrilling AND compelling.
And that's not even going into him fighting the magic high commission like Toffee before him, and how his new interactions with Hekapoo would be like, ESPECIALLY since Hekapoo always had a feeling magic was bad, and saw Glossaryarick working with Marco in the most honest fashion she has ever seen. Hekapoo may actually straight agree with Marco, but fight him in the name of the current queen of Mewni, or she might turn coat and work with Marco as a whole. She has defied the magic high commission a lot in the show, and shows she's really loyal to those she calls friends, so we really could've had a crazy story.
God I want to buy the rights for this show, but I have no idea how much funding I would need to do so.... But I would definitely go down this path.