I actually don't see Modern AU Hürrem as a college-going type at all! In my long-abandoned fic, I made her a maid/housekeeper (to Hatice - which is how she meets the big shot executive nepo baby Süleyman lol) who has dreams of being an actress/model. Like Modern AU Hürrem wants to be famous, seems to have some acting talent (although she is still a down-on-her-luck wannabe) and has neither money nor really interest in any subject. Her father is really dissapointed ofc - if he's alive, that is.
Süleyman: As mentioned, he's a nepo baby par excellence, probably has a major in econ or business or something to that effect. He's cultured, well-educated and has interest in history and literature, but he didn't study it, in part because of his father's expectations, in part because he didn't want to. The other option is that he joined a military academy of some sort and now is climbing the ladder in that field.
Ibrahim: Whatever Süleyman studied, so did he, either they met in college, or their interests are similar enough that he followed Süleyman to his college of choice, possibly with the financial support of the Osmanoglu family.
Mahidevran: She either went to college to get a Mrs. degree, or studied the same thing as Süleyman, just wasn't as successful as him. Either way, she probably dropped out after marrying Süleyman and getting pregnant with Mustafa, and didn't regret it very much. She's more or less happy as a rich housewife and socialite.
Hatice: A lit major, and damn passionate about it, too. How successful her academic career is I am not sure, but I'd like to think she does at least strive for one.
Ayşe Hafsa: Also dropped out of college to get married, but that was less out of her own volition and more because in her generation, it was kinda expected of her. She might've studied literature, just like Hatice, creating some interesting dynamics between her and her daughter.
Nigar: I made her a cop in my Modern AU, but she could also be the Osmanoglu family doctor or secretary of some sort. If she went to college, it was probably medicine.
Sümbül: Some kind of vallet or secretary for the Osmanoglu family. From a poor family, probably didn't go to college either.
Şah: Now THAT'S a religionistic girlie if I've ever seen one! Much like Hatice, she has an academic career, on which she collaborates with Lütfi, until he gets into politics.
Fatma: Either an actress or a romance novelist, successful or otherwise. Possibly both at different periods of her life. If she went to college, she only did so on her family's insistence and didn't have very good grades.
Mustafa: As I've said in one of my previous posts, this is an army guy trough and trough. Military academy.
Mehmet: Probably followed Mustafa to military academy, because he doesn't have many ideas of his own. Failing that, uncle Ibrahim might've talked him into classical lit, since he seems to have some interest in that in canon.
Mihrimah: Okay, I am kinda at a loss here. She probably did go to college, just because I feel like Modern AU Hürrem would pressure her into it (instead of marriage to Rüstem), in part due to her class inferiority complex, but whatever they'd pick, Mihrimah wouldn't be very enthusiastic about it. My guess is actually law, just because of the prestige involved, and also because as a lawyer, she can play the same auxilliary role in her family (as a law consultant for the family firm) she did in canon.
Selim: Dunno, art or something. That, or some major his family pressured him into, like business or law.
Bayezit: Tries to go to military college, but fails. Ends up also studying art, but in a college on the other side of the country, just because he wants to avoid his brother that much.
Cihangir: Something history or literature related, much like Şah and Hatice, he ends up doing it professionaly.
Nurbanu: Astronomy, naturally!
Huricihan: Do I even have to say it? Musical college ofc.
Mihrunissa: Went to military academy with Mustafa.