Well, I don't think it would or could be anything physical (human + fae normally results in a Changeling, not a Spirit of Intellect).
And I don't know that Mother Winter is worse than an actual Fallen. There could be a Shadow of some sort involved, just as with Harry/Lash.
Also, just because Mother Winter has never retired, she wasn't necessarily always what she is now. If the Faerie Courts had a major realignment around the time of Hastings, her role/Mantle (and therefore her nature) might have been rather different before that.
I wasn't suggesting a physical act of copulation...yeesh! More that Mother Winter isn't really a person. It's like connecting your mind and soul to destruction itself. Like connecting your brain to a nuclear reactor. It isn't going to go well.
The changeling problem is important. But that doesn't rule out a Fae parent either.
True, true. Hard to say what's worse. I suspect it wouldn't matter much as either Fallen or MW you would be equally dead (although you could argue about the damage to your soul). Depending on who she was might be the missing link.
I think the most important thing though is the statement about it being 'obvious'. To me that implies that not only does it have to be one immortal or ancient supernatural being and one semi-mortal (but old enough to qualify), but that there has to actually be some relationship or connection between them established.
The only ones I can think of that fit that are Odin / Gard, Merlin / Demonreach (if Bob's movie in CD qualifies him as 'appearing in the series'), and - at a stretch - HWWBehind / Lord Raith (but I *really* don't think Bob is half-Outsider).
There aren't that many semi-mortals over 1000 years old in the series, and a lot of them are Denarians who probably aren't qualified (any "shadow" they had would have been re-absorbed when they took up the Coin fully, so no chance of a Harry / Lash type situation).
River Shoulders has no established relationship with an immortal.
Bob didn't know Demonreach, and doesn't have a particularly Norse theme. He also didn't recognise Merlin's magic. So that should rule out most of that. I certainly don't see a particular White Court or Outsider theme. Harry would surely have recognised that!
Truth is the hints we have are many. But being a spirit of Air and intellect (and the fact that it is a known character we have met) almost certainly means Mab is his mother. It's whether she was Queen of Air and Darkness at the time, or whether she was merely the Lady or even mortal herself. Because all of that would decide whether Mab was the mortal in the relationship or whether she was the immortal.
So really we have to figure out the father. I think an If x, then y formula should do.
If Mab was mortal, then Vadderung (or one of his aspects) is most likely.
If Mab was immortal, then Merlin would be most likely.
However there are a few things that stand out.
Bob acts mostly like a horny teenager. So Harry has a had a strong effect from his youth. But when carried by Cowl or Corpsetaker he acts cold and ruthless and sadistic...but always somewhat insouciant.
So in someways, the mortal he resembles most, is Id-Harry.