I really wish T'Rul had stuck around, especially after Worf came onboard. I like to think there could have been a good love/hate relationship between the two of them.
I'd have liked to have seen Dorian Collins assigned to the station, she could be a love interest for Jake or Nog. She could have been the Defiant
's conn officer after Jadzia was killed. Some survivors guilt to work through, as well as living up the legacy of Red Squad (seeing as she is the only one left).
A Federation diplomat/liaison. He/she would have their own approach to alien races, interested in peace above all else and has a dislike of Starfleet due to its militaristic ways.