These Things Take Time (READ DISCLAIMER)
6 parts Complete Disclaimer: All rights go to dearzoemurphy on AO3 who gave me permission to share their work
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Natasha Romanoff is content with her life at the moment. Not much has happened since she moved into the Avengers compound, and she's okay with that. She spends most of her time helping the newest Avenger, Wanda Maximoff, hone her raw strength, which is no easy feat. Natasha's happy to help until both of them start to have a few unfortunate realizations. Like maybe that twinge in Natasha's gut is jealousy, and maybe Wanda has a thing for women who could kill her a million different ways.
(Slow-ish burn, post-Age of Ultron, takes place between AOU and Civil War when the six new Avengers are living together in the compound.)