I don't like the idea of Kang being descended from Reed and Sue. It's a locked in future that seems a lot less bright than the FF usually has. I can get behind a Nathaniel Richards ancestry though. He had kids all over.
As to Franklin, his post Hyperstorm interpretation has him as very long lived as per Hickman's run and Waid's History of the Marvel Universe. The thought of him having children is very possible. Valeria's a blank slate. I'd imagine she's already started a genetic study to see what her offspring with various characters would be like and their power sets.