Well, this is one of those ships where I think the canon pairing is so strong that most fanfiction naturally sticks close to it. The beauty of Mai and Sakuta is in their established, mature dynamic, so the best stories for me are ones that deepen that, not replace it. I've read a few popular AUs, like coffee shop or university settings, and they can be cute, but they often lose the specific melancholy and sharpness that defines their relationship. The real standouts explore the 'what-ifs' within their own world: a story where Sakuta's scars from the adolescence syndrome manifest differently, or one where Mai's career forces a long-distance arrangement and they have to navigate that adult stress. There's a lesser-known one called 'Ad Libitum' that imagines them years later, dealing with the ghost of Shoko's timeline still hanging over them, and it felt painfully in-character.
Honestly, I'm not a fan of most crossover pairings for them; putting Mai with someone like Hachiman from 'Oregairu' just feels like mashing two 'loner' types together without the chemistry. The only exception I've ever enjoyed was a surprisingly thoughtful fusion-fic where the 'Adolescence Syndrome' concept bled into the world of 'Steins;Gate', and Sakuta had to work with Okabe. Mai's role was smaller, but her pragmatic acting skills were used in a brilliant way to ground the science-fiction chaos. Mostly, though, I'd say skip the pairing swaps and look for authors who really get the original voices. The best fics make you feel like you're watching a lost episode.