How do I ask my roommate if she wants to live together next year? I think we get along great, but she hasn't brought the idea up."
I love questions with simple answers.
You say "Hey roommate, I think we get along great, what do you think about living together next year?"
If she says yes, buy some celebratory wine and enjoy the adult life.
If she says no, don't take it personally (unless of course, she makes it personal: then buy some wine anyway and be thankful you don't have to live with them again. Also remember that outfit they wore that one time. It looked so bad. I mean, come on, you didn't want to live with them anyway.)
But if they were decent and just said sorry but they're rooming with their study group, or aunt, or whoever, don't take it to heart. You can get along great, but that doesn't mean you're great roommates. You can be great friends and have perfect schedules that complement each other and never get in each others' way, but that still doesn't make you ideal roommates.
Maybe you like to chat about your day (I know I do) but your roommate just wants to sit in silence for an hour after class. Maybe you love, love, love tuna sandwiches but your roommate just wants to hurl every time you crack a can open. Living together means spending a huge amount of time together and that can lead to people only seeing your weird habits you might not even realize they hate.
Take myself for example, I have three amazing roommates and I've never once had an argument with any of them. We get along fine and chat when we see each other, but does that mean I'm going to live with them all next year? No. They're great, but no. I'm sure they feel the same way about me. They probably hate that I run the dishwasher at night (peak hours people) and that I watch television when I bake, which is often. That doesn't mean they hate me (or at least I hope it doesn't) but it could be enough to make them look for a new roomie for next year.
What I'm saying is if your roommate passes on a chance to live with you again, then that's their loss. Clearly you're a great roommate and decent person, if a little insecure, so your next roommate might be an even better fit. Who knows your next roommate could be your one true best friend.
Finally, as a general reminder, you don't need to be good friends with your roommate to be a good roommate.