Yes, there are many. You can start by looking on popular fanfiction websites like Archive of Our Own. There, you'll find a wide variety of stories exploring their relationship. Some might focus on a different take of their interactions in the show's universe, while others might create entirely new scenarios for them to fall in love all over again.
The appeal lies in the fact that they had an interesting dynamic in the show. They were on opposite sides initially, which adds a layer of tension that fanfiction can explore further in a romantic context. Their different personalities also make for an exciting combination.
Definitely not. Fanfic should generally focus on positive and respectful interpretations of a character or a person's work. Sexual fanfic about Alison Brie crosses the line into inappropriate and unethical territory. It can also make the actress uncomfortable and is not in line with proper fan behavior.
Love is a very common theme. Since their relationship has a strong romantic element, fanfictions often explore different aspects of their love, like first love or rekindled love. Another theme is friendship, as they were friends before becoming more. And then there's the theme of acceptance, especially in terms of their relationship within their community.