You can start by looking on fanfiction.net. It has a large collection of fan - made stories, including many Kara Lena ones. Just use the search bar and type in 'Kara Lena' and you'll get a bunch of results.
One great Kara Lena fanfic could be 'The Bond Beyond Powers'. It delves into how Kara and Lena's friendship blossoms into something more in a world full of challenges. They face villains together and in the process, their relationship deepens on an emotional level.
I'm not sure specifically which 'Kara and Lena rated M fanfic' are the best. You could try searching on popular fanfic platforms like Archive of Our Own. Look for ones with high ratings and a lot of reviews from other readers.
It can be portrayed as full of mistrust. In the nightmare, they might constantly suspect each other of having ulterior motives. For example, Kara might think Lena is secretly working with her enemies, and Lena might think Kara is hiding something important from her.
One possible scenario could be Kara losing her powers in the nightmare and being unable to protect Lena from some unknown threat. Another might be Lena turning against Kara due to some false information in the dream world.
One possible plot could be that Lena and Kara are in a dangerous situation together. As they face a powerful enemy, they realize their deep feelings for each other. In a moment of intense emotion, perhaps after one saves the other's life, they share a kiss.
One possible fanfiction could be about Lena Luthor becoming a mentor to young Kara Danvers. Lena, with her intelligence and resources, could help Kara understand the complex world around her while also learning to control her powers better. Maybe Lena discovers Kara's identity early on and decides to guide her in secret, away from the prying eyes of Lex Luthor and other potential threats.
One idea could be that they overcome some external threat together which makes them realize their love is unbreakable and they decide to get married. Maybe they fight against an anti - alien group that targets Kara, and Lena uses her intelligence and resources to protect her. This shared experience brings them closer than ever.
During the pregnancy, their relationship could transform in many ways. They would become more empathetic towards each other. Kara, with her super - strength, might be more cautious around Lena, not wanting to accidentally harm her. Lena, on the other hand, would use her intelligence to research the best ways to have a healthy pregnancy for both of them. They would also have a new shared purpose in life, which is to raise their children well, and this would create a deeper and more complex relationship dynamic than before.