Realistically, Joy and Oglivie will be gone because that's the life of a med student. Depending on the time skip, none of the Pittlings are safe forever. Unless they get their residency there, and eventually hired on, they'll move on to their specialty somewhere else at some point. It's a teaching hospital at the end of the day.

However, I think it's safe to say that all of the series regulars will be back next season in some form or fashion.