Here's my question. What do you think would change, if Snape was female?
Because... like... I don't think it would change much? His family trauma doesn't seem to have a specifically *gendered* component... his bullying from James - maybe does? Like James might bully a Fem!Severus differently? (Or then again, maybe not.) Fem! Severus would still be weird, poor, dark arts/slytherin aligned, not attractive (to James)... but also a big draw on Lily's time and affection in a way that James wouldn't like.
I don't think it would changed his relationship with - Dumbledore, the Malfoys, Lupin. Bellatrix might... possibly hate a Fem!Severus more? Like see her as more of a rival? But she already does seem his as a rival, so. I'm not even sure a Fem!Severus would dress or wear her hair any differently.
Very interesting ask. šļøšļø I agree that not much would have changed but I do see a few little things here and there which, once you addition them, could make her act a little differently...
Childhood
She would still experience a pretty miserable life at Spinner's End, but she might have felt a bit less awkward with wearing her mother's clothes, thus making her maybe a bit less socially stunned.
She would still meet Lily and not get along with Petunia.
Hogwarts
You pointed out the most important and obvious point already: the bullying. Not that it would have not taken place - guys can absolutely bully women - but considering the time period I suspect Sirius and James's pureblood upbringing would have influenced the way they interacted with Fem!Severus.
First, their first encounter on the Hogwarts Express might have been less aggressive. I think James and Sirius would have been taken aback and super weirded out by this poor, unpleasant and ugly girl, but I don't think they'd have insulted her as harshly/tried to make her trip or nicknamed her. Maybe some things would have been salvageable then - not talking about friendship here (pretty impossible) but just... being civil?
It's pretty certain James would have still been jealous of Fem!Severus for hanging around Lily but he wouldn't have needed to prove he had the bigger pair - which was a good chunk of what it was in canon imo. He would still tease her and ridicule her verbally in front of Lily but maybe less viciously. Fem!Severus would still fight back so this could escalate to jinxes but still, it has the potential to be way tamer overall.
As for Sirius, I think he could have seen some of his mother in Fem!Severus, which could have resulted in either: wanting to bully her even more OR unconsciously trying to avoid her. I can see her making him super uncomfortable. I'm personally leaning toward the latter option. A man he could be violent with - no problem - but a woman? Hm. I'm unsure.
If the bullying was lessened, then I don't think it would go as far as the Prank or SWM, because pantsing a girl, even one you really don't like, even one who might be interested in the Dark Arts, seems super extreme coming from two pureblood boys. The 'boys will be boys' excuse functions better between all boys in this instance. I think this would have felt ickier for everybody around them. Tho if the Prank did happen, Dumbledore wouldn't move a finger either, that's for sure.
No SWM means no 'mudblood' incident. This being said, I don't think her relationship with Lily would have changed much. What drove them apart isn't related to gender and it's pretty clear Lily wanted to distance herself anyway. SWM was an excuse so she would find another one.
Also less bullying doesn't mean she wouldn't become a DE because she would still crave a place and be susceptible to Riddles' promises.
First War
I do wonder how it would have changed her relationship with Lucius Malfoy tho? As a young man he may have kept a bit more distance between them as to not start rumors? So maybe she would have gotten less support from him? But maybe she could have become friends with Narcissa and it would have been the same (I like that idea).
Dumbledore would have maybe been less harsh with her when she first came to him or when the Potter died, since he would maybe project less of himself and Gellert in her... but he would definitely have still manipulated her the same way. Dumbledore is a lot of things, but he's not sexist.
Harry
Later in life, since she wouldn't be as traumatized, she would have been less triggered by Harry's resemblance to James, thus making their relationship less disastrous. She would be cold and distant but less cruel. She would also have been able to teach him Occlumency more successfully, which could have changed quite a few things plot-wise...
Second War
I also agree with the idea of Bellatrix being jealous. I really see it. She would certainly try to slander her/hex her viciously in order to get rid of her. This would have maybe made Fem!Severus' work way harder. Maybe this would have resulted in her trying to get rid of Bellatrix.
I wouldn't be surprised if during the Second War many DE thought Voldemort and Fem!Severus were somewhat of an item. He was pretty fond of Severus after all... and if Bellatrix proved to be too annoying and Voldemort ended up getting rid of her it would rightfully start some rumors. Maybe Dumbledore would have tried to encourage it even? I don't know if that's really his style but I wouldn't get it past him to suggest it at the very least.
Conclusion
So... yeah. I don't see this many changes (I may have overlooked some things). Severus is already pretty female coded in many ways (his writing, the way he talks, moves, his interest in potions, the way his (very valid) feelings are dismissed etc.) and in a Fem!AU, many interesting things could be explored in the way people would interact with her.
But I do think she could be less traumatized and more self-assured. Maybe she would have enough mental energy as an adult to create spells and potions. She wouldn't dislike Harry as much and maybe have a bit more self-preservation in regard of Dumbledore's plans for her. I don't see her being ready to sacrifice herself as fully as canon!Severus. She'd have a few backups.
But she would still wear a grey nightshirt.
And have pretty perky little boobs.
