I remember seeing a theory post a while back about the identity of Dean Thomas's Father, since we know it was originally going to be revealed at some point, and the person specifically argued that since Dean is always put in proximity to the Black family, and that his father was someone who believed himself to be at risk that his father was possibly Alphard Black, or a descendant of Alphard Black. I do kind of like this theory, and if Alphard Black had a child out of wedlock that might explain why they don't appear on the Black Family tree. I wanted to figure out a fanon-identity to use for this character, since what we know of his characterisation is actually very interesting, where he married a muggle woman and even had a child with her without ever revealing he was a wizard, and then left when the war began to intensify out of fear for the trouble he would bring to his family, dying because he refused to take the Dark Mark, clearly meaning the Death-Eaters wanted to recruit him. I tried to figure out a relatively important Black wizarding family, and all I came up with was the Shacklebolt's (*cough* Sterling's *cough*). And no I'm not biased because Kingsley is one of my favourite characters, shut up.
So, Alphard leaves his family at some point and lives with Aurora Sterling, and the two never get married but they do have a son together named Marius Sterling (Marius is a name that does appear on the Black Family tree once, so I figured why not use it for a second member of the family). Marius attends Hogwarts, although I don't think he'd really reveal himself to the Black family, since Alphard Black is a surprisingly prominent but very mysterious character, so I assume he's kept his life hidden from his family. When Alphard dies, most of his wealth goes to Aurora and Marius, with a portion of it going to Sirius to help him escape the Black's. Marius falls in love with a muggle woman, they have their story, he leaves and the Death-Eaters, specifically probably Bellatrix and Rodolphus I imagine, go after him after in some way, shape or form, figuring out who he is. When he refuses to join they engage in a duel, and one of the two kills him, although they're not even sure which of them landed the final blow. (This makes it easier for Bellatrix to spill more family blood later down the line). The reason I think specifically Bellatrix figured it out, is because Bellatrix is the member of the Black family Dean is placed in proximity to, and if his parentage were to be revealed while hostage at Malfoy Manor, it would likely be her doing the revealing. I also picture Marius Sterling's death being a big motivator in making Kingsley join the Order, since that's his cousin.
Boom, my version of Dean's Dad explained. A friend (@/DBR_Augary on Ao3) also came up with names for Dean's Stepdad and half-siblings, Mitchell Jameson, Mary Jameson and Walter Jameson, so do with that what you will.
