My personal theory is that perhaps Isshin is a Shiba? Maybe an Uncle or other relation to Kukaku, Kaein, and Ganju. I always thought him and Ganju kind of look alike, maybe thats just me, and it could explain why Ichigo and Kaein look almost identical. I've seen cousins in my own family that look enough alike they could pass for siblings. As for his rank I'd say he was probably a captain. In the anime, I haven't read the manga yet, it looks like he has a captain's robe hanging from his shoulder. I can't really fathom a guess as to how old he is or anything about his powers. I'd like to guess that maybe he was in squad 4, he is a doctor after all. As for Ishida again I can't say. I'd have agree and think that maybe there is some relation between Masaki and him but again not positive. Isshin really is still a mystery.
Edit:
Also on the idea of Ichigo being half shinigami/half hollow. I have to ask: What about Karin or Yuzu? They really don't seem to play any important role that I've seen so far but they are Ichigo's sisters and in the beginning Karin, correct me if I'm wrong, could see speak and touch ghost just like Ichigo. I guess there could be some twist where they aren't really related after all or maybe there is just simply that there is a differnce in levels between them and Ichigo. I dunno just thought I'd say it.