What if Qrow is Ruby's father?
Here's how it plays out in my head:
Taiyang and Raven paired up.
Summer and Qrow paired up. Thing is, Qrow is less responsible, so Summer and Qrow's relationship wasn't going to marriage anytime soon.
When Raven dissapears, Qrow (quite responsibly) up and dissappears to go look for her, without a word.
Unbeknownst to him, he's knocked up Summer.
Summer and Taiyang become an item: he helps her through her pregnancy, she helps him with baby Yang.
Qrow comes back and has a Bill at the wedding moment.
I realize that this would add unneccessary melodrama, but it would be some interesting moments of character development in flashbacks for Taiyang and Qrow. Both would be quite over thier past by the time of the show. I can imagine Qrow telling Taiyang and Summer that Taiyang would be a better father for Ruby anyway.
I explains why she is Ruby Rose and not Ruby Long: Taiyang and Summer weren't married when she was born (and they didn't bother to change her name [Independant of her father, that is probably why she has her mother's name]).