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.

The biggest difference, I think, is that the Master and Lucy are much more brittle/fragile than the Doctor and Rose. Where the Doctor and Rose can bend and then snap back into place, the Master and Lucy break.

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.

This show has so many interesting things like that to think about, because the characters have so much depth and life to them.