one idea for a hp fanfic - that will never be written, as there’s no way it can work without some characters being very ooc - is Narcissa and Sirius somehow geting stuck together in grimmuld place during order of the phoenix. This will never happen, but the idea has so much potential.

imo Sirius and Narcissa have one of the most interesting black family dynamics to theorise about, because there’s so many interesting ways you can take it. So much angst and potential for tension/anger/feelings of betrayal etc. Some ideas for this scenario;

- Narcissa arguing with Sirius about his complete lack of regard for their house, throwing away ancient black family heirlooms etc.

- Sirius blaming Narcissa for Regulus’ death - my headcanon is, whilst Sirius spent time with the Potter’s, Regulus went to the Malfoy’s. In Sirius’ mind this allowed Lucius to get into Regulus head about Voldemort - what Sirius doesn’t know is Narcissa begged Regulus not to join (even Lucius - more subtly - tried to convince him not to).

- Sirius does a lot of passive aggressive things ie deliberately gives Narcissa Regulus’ bedroom, kept exactly as is, as a way to punish her. Eventually it cultivates to the mother of all arguments.

- Sirius calling her Cissy. I don’t know why but this just makes me go feral.

- back in the day, Narcissa’s father wanted Sirius and Narcissa to get married (relating to his own insecurities, feeling second best to his sister, not being the heir etc). This wasn’t a very popular idea, even amongst the black family first cousins marrying was.. a little too far (and I don’t think traditional arranged marriage was a thing - it was more like, here’s 10 approved people, you choose) but he was really trying to push it (if he can’t inherit, at least his daughter will). Narcissa knew this but Sirius never did. He was 4/5 years younger and by the time he would be old enough to know, it was never going to happen anyway. Sirius reaction would be hilarious if this came up somehow lol.