Hope this is where to discuss rumors and what-not and I also I hope this hasn't been discussed on here yet, if so I apologize.... Here's what I've heard:
McG is related to Lucius Malfoy. He is a cousin of hers once removed. She's Rodolphus Lestrange's aunt.
Not sure if this is true. Anyone know? I found said information awhile back while browsing various HP forums and I can't remember where it was I saw it. I have no idea where the person that said it got that information from.
Then I read somewhere that in the movie version of Sorceror's Stone when Harry checks the plaque to see his father's name on the quidditch team as a Seeker, one of the other names on that plaque is I *think* M.K. McGonagall. I know the last name and first initial is correct, I may be wrong about that 'K' though.
Soooo, this could imply that McG had a kid that went to school in the same era as the Marauders, and played quidditch with Harry's dad! **awwww McGonagall mommie! Cute!!*

Buuut, that brings up a few things to ponder:
A.) who's her kid?
B.) is said kid still alive?
C.) who'd she marry (um, *if* she married)?
Hmmmm....