Or you know, you could actually be a decent villain, like Doctor Doom or Dormammu.
Oh, here is another thought - Mordo is only a threat when he is backed up by stronger antagonist. Is that what you want for Nico? To be a joke who needs to pledge herself to bastards like Dormammu or Shuma-Gorath to actually be able to accomplish anything? Walking "No Self-Made Woman" trope?
She haven't been using this name outside one 4-issues story. it's horribly dumb to make her a villain because her nickname should villainous. I like Nico partiarly because, despite her dark attire and powers, she is kind, good person. Making her villain would take it all, alsongside many other things I love about her, like her motherly nature and compassion, away, thus making her not interesting at all.
I often associate creepy with physical/descriptive traits, and tend to understand it as "unsetting " and "unpleasant to look at", I apologize for the confusion.
Personally I see future Nico as a hero. Maybe not 100% goody-two-shoes, maybe more willing to cross few lines, but still a hero. More like well-written Doctor Strange.
Master Cyclonis from cartoon Stormhawks. She is pretty much everything evil Nico could ever be and more. And as I watched that cartoon and Cyclonis is my favorite character from it, I have no need or desire to see another character I like turned into cheap knockof of her.
Marvel has loads of villains, half of which they never use, especially magic ones. When was the last time Umar did anything trurly villainous? Half of Strange's and Thor's Rogues lie in limbo unused.
Second, most of Marvel's magic users are either dead or evil. This is why Marvel had always been losing at their magic with DC, why until Kieron Gillen came along they never produced anything you could honestly call their equivalent of Sandman. Marvel is obsessed with Science-Fiction and tends to dismiss magic and those who use it as "these arseholes who groans at our cool sf heroes from other dimensions". This is why I hate Jonathan Hickman's New Avengers too, with him reducing Doctor Strange to a dead weight and having plot about how he sacrifices good side of him to become boderline villain for good intentions (AGAIN!).
And yes, you can have too many witches just, like you can have too many of any character. If 90s taught us anything, with their army of Cable clones, is exactly that.