The Doctor always seems to have his "main" companion at any given time. Right now it's Clara. Some of the Doctor's companions have been given permanent endings. Amy and Rory got trapped in the 30s. Donna can't remember the Doctor without her head exploding. Rose is sealed off in another dimension.
But then we have characters like Martha and Jack Harkness. Martha chose to leave the Doctor at the end of her season, but she was still alive and well. She started working for UNIT and popped up a couple times in Season 4.
It got me thinking; I'd like to see more of this, and I'd like it to start with Clara.
Even if Capaldi's Doctor has a new fulltime companion once Clara leaves, I'd like her story to allow for the possibility of her return. She already doesn't live on the TARDIS like other companions. The Doctor has adventures with her and drops her off at home. She has a life and a job away from him.
I guess I'd just like more open-endedness with some of the companions. I'm getting a little tired of these permanent, tragic finales where the Doctor can never, ever see his friends again.