I was just thinking about this stuff last week after rewatching Last of the Time Lords. I find the connection between Doctor/Rose and Master/Lucy really fascinating. I think watching Master/Lucy in action tells else alot about the Doctor/Rose relationship and visa versa. The funny thing is I believe that given one or two changes to their lives/personalities, the Master is capable of doing everything the Doctor is and the Doctor is capable of doing everything the Master is.

I never had thought about the yin yang of Rose and the Master when it comes to how they effect the Doctor though. I totally see were you are coming from. Very interesting.