I wouldn't mind seeing Lady Christina De Souza back again either. I thought she was great in Planet of the Dead. Enigmatic and naughty, much like the doctor, and rather than clashing, their similar personalities worked well together.
She wanted to join the Doctor, but he stated that he was too hurt at losing previous companions, and didn't want to lose another. He seemed to get over this with his latest regeneration, so maybe she could come back.
I really like this suggestion, fully agree.
My two criteria for the next companion(s) is simple;
1. No romantic guff with The Doctor
2. The arc does not have to be all about the companion(s) every year.