I'm not convinced that we've seen the last of Murphy in this series. Yes we've been told that it's against the Rules for her to come back any time soon, but Dresden isn't exactly known for letting Rules stop him. Also I suspect that many, many Rules are going to become obsolete once the capital A Apocalypses get under way.

So, by now we know that Mab was linked to Merlin, and that she loved him. It probably means, following the Arthurian legends, that she was Niniane/Viviane when she was a mortal, and that she is somehow responsible for Merlin's death/imprisonment in Demonreach.

On the Bayeux Tapestry there is a famously mysterious scene of a cloaked man touching (slapping?) a woman. The scene is titled "where a certain cleric and Ælfgyva." The name Ælfgyva means elf-gift. When Corb was taunting Mab about Merlin turning her away, my first thought was that in the Dresdenverse this scene is a depiction of Merlin casting out Mab.

Jim has said that he thought it was obvious who Bob's parents were. So if one is Mac, is the other Mab? Who else do we know from back then?

That's my gut feeling. I don't have any direct evidence, but this exchange from Skin Game is what makes me think they have a past relationship.

Mac, bald, lean, and silent, stood behind the bar in his usual crisp white shirt and spotless apron. When Mab entered, he put down the rag he was using to polish the wooden bar, and bowed at the waist, somehow giving the gesture an accent of courtesy rather than obeisance.
“Barkeep,” Mab replied, and inclined her head considerably more deeply than she had to Nicodemus a few minutes before. “May your patrons be prosperous and honest.”
Mac, as a rule, rarely uttered multi-syllables. Today, he said, “May your scales always return to balance.”
Her mouth quirked at the corner and she said, “Flatterer.”
He smiled and nodded to me. “Harry.”

These are normally two of the most reserved characters in the series, but here it seems like they are flirting. It isn't just Mac's sudden loquaciousness that strikes me as unusual. Mab's response is her equivalent of a teenage girl blushing and giggling. I can't recall either of them behaving quite like this with anyone else.

Ever since Skin Game, my pet theory has been that Mac is Bob's father. Mythology says that Watchers are angels who produced offspring with humans. In the Dresdenverse angels are beings of pure soul, and we are told that pairings between spiritual entities and humans produce spirits of intellect. In mythological terminology Bob and Bonea are Nephilim.

So my scenario is that about 1000 years ago, angelic Mac fell in love with a human woman and they produced Bob. That caused him to fall from grace and transubstantiate into his current form. He's probably in Chicago to keep an eye on his son.