The nonsense about the wasteland world (does it have an official or at least commonly used name?) not having many women because of the wars, makes ideas pop up. If there aren’t a lot of women, then there probably aren’t a lot of children being born.
We don’t know if Cas exists in the wasteland and whose side he’d be on. I always think that Cas, in his very core, cares too much, so even if he would fight alongside Michael, he’d be very opposed to the strain on humanity this whole battle has.
The earth is being destroyed, humanity is dying out. Surely that’s not something Cas - in any universe - would be particularly okay with.
Which brings me to this thought: imagine Cas - using a female vessel, maybe never having “lost” the one he used in 1901 - working to build a sort of sanctuary for humans. She would take care of the children, maybe even have some of her own. As an angel she won’t be harmed by pregnancy, nephilim pregnancies last a shorter time and if you extract the Grace of the child beforehand, you’ll get a human kid. (Maybe they would use the Grace to bring some sort of life back into the wasteland.)
So this is where my mind at is right now. And if I think it through further then of course I want her to meet normal universe Dean and decide that she must have this man’s baby. *lol*
(Of course, nobody is surprised that my mind went there.)