I thought they were symbiotic beings, but what one feels, feels the other as well. It broke my heart in Fallout 3, I decided to let him live for selfish reasons, them being that I knew him and Bob for so long. I just could not do it. Besides he was not aware what good he caused in Oasis so this schould be preserved with him.
I doubt they can bring him back though. Not without a rather illogic excuse to make him move about, and not without making a quite heavy moral decision canon. Have they done that before?
I always want to meet characters form previous games, but only ones that stuck in my head. Dr. LI might make an appearance, but honestly I wouldnot care for her. It was nice seeing Marcus in NV. As for Fallout 1 or 2 characters I doubt we could see anyone that is not a Mutant or Ghoul, thirty years is a long time in the Wasteland. But there are few memorable people left.
Thinking about it, NV had quite a few, Cassidy's daughter for example, who was exactly like old Cassidy, well without the weak heart, and Dr. Henry who was rather more sympathetic than in Fallout 2. And Jacob of course. Any character from these games would have had a long journey to wander.
Apart from Dr. Li do we know of one person that might have travelled to the Capital Wasteland and beyond?