10. The Halliwell Sisters

9. Scarlet Witch

8. Nico Minoru aka Sister Grimm

7. Zatanna

6. Bellatrix Lestrange

5. Wiccan

4. Fred Armisen as Joy Behar

3. Willow Rosenberg

2. Nancy Downs

1. Professor Minerva McGonagall

Because this season could use a few more competent witches.
I mean, they already taught at a school (cleverly titled Magic School) that was not unlike Hogwarts on their show, Charmed. Also, they did a pretty good job of vanquishing demons with the flick of their wrists. Also also, I'd love to watch Rose McGowan's Page Matthews exchange a few insults with Emma Roberts's Madison Montgomery.
She's had quite the career of changing from good to evil and back again. She's crazy powerful, has loads of family issues and sometimes can't control her powers--you know, like that time she said, "No more mutants" and nearly all mutants lost their powers. She'd do great on this show (and would, no doubt, cause a murder or two once she went all Dark Phoenix-like).
The sorceress daughter of evil dark wizard parents, she can cast any spell imaginable when using her Staff of One (but each spell can only be cast once and she can't bring people back from the dead) and is the unofficially leader of the team The Runaways. She'd be a great goth girl addition to the Coven. I'd like to think her and Taissa Farmiga's Zoe might get along.
A very powerful sorceress who wouldn't take herself too seriously. She could maybe even have Wonder Woman stop by and give a guest lecture at Miss Robichaux's Academy.
She's the villain you love to hate. Also, she'd definitely be able to take on every witch in this coven. She'd make for an even better villains than...whoever it is you want to consider a villain this season (it might be everyone--I'm not sure).
I have no clue (and would guess that neither does Mr Ryan Murphy) what the rules are on a male in the coven. But, Leslie Jordan was on The Council of Witchcraft…so, I'm thinking they're, at least, gay friendly? Anyway, Miss Robichaux's Academy could use an angsty teen gay who happens to not be evil. He has a really great backstory (was maybe not ever really born, has a twin brother who also has powers, his mother is the sometimes insane Scarlet Witch, etc) and would make for great TV. Also, he has a really great superpowered boyfriend.
Hear me out! She'd make a great new neighbor now that Patti LuPone's crazy-enema-giving-Christian-mother character is dead. Fiona Goode would be wondering around, rubbing her neck and plotting how to kill everyone while the girls went next door to have a glass of Pinot Grigio with their new neighbor, Mrs Behar, who would give them some sage words of advice, "So what? Who cares?"
She'd, at this point, make a really great Supreme. At the end of Buffy, she was in control of her powers and awoke every Slayer. Also, she'd definitely be able to take on Fiona Goode. She's more powerful than all the other students at Miss Robichaux's Academy. So, either she could teach them or go evil and, ahem, slay them all.
In The Craft, she invokes a deity, Manon, and kills her abusive stepfather with her powers. I can't decide if she'd be better as a student or as a villain. But, if the writing on this show has taught us anything--it's that it doesn't matter. You can pick and chose what episodes your characters get to be villains in. She might fit in better at Miss Robichaux's Academy than the rest of the people on this list. She'd definitely end up slapping the crap out of Madison and making all the other girls live in fear of her.
Let's face it, Cordelia Foxx has been a less than stellar headmistress. Students are being murdered inside her school (which is significantly smaller than Hogwarts) nearly every other day and she hasn't been able to do anything about it. Professor Minerva McGonagall doesn't tolerate crap even from some of her best students (cough cough Ron Harry and Hermoine cough cough) and also has proved that she can hold her own when her (much larger) school is being attacked. Maggie Smith's Professor McGonagall, queen of the side-eye, would be perfect as the new headmistress to teach this bratty coven a thing or two. She's the headmistress Miss Robichaux's Academy needs and deserves.