Yes, in many fanfictions, Damon and Elena do get back together. Fanfiction writers often explore their relationship further and give them a second chance at love, as many fans were disappointed when they didn't end up together in the original series.
Often, in fanfiction, there's a time - jump scenario. After a period of time apart, both Damon and Elena have grown and changed. Damon has become more self - less, perhaps having worked on his issues with his brother. Elena, on the other hand, has realized that her feelings for Damon were deeper than she thought. They then meet again by chance and start to rekindle their relationship. Another scenario could be that a magical or supernatural event forces them to work together. During this time, they remember all the good times they had and fall back in love.
Stefan could be really interesting. Since he has a long history with both Elena and Damon, his reactions to their marriage would be complex. He might be a bit jealous at first but then come to fully support them, adding an emotional depth to the story.
One possible plot could be that after many trials and tribulations in their relationship, Damon finally proposes to Elena in a very romantic way. Maybe he takes her to a place full of their memories, like the waterfall where they once had a deep conversation. And then they plan a big, but also very private wedding, only inviting their closest friends. The wedding could be a mix of modern and traditional elements, with Stefan giving a heartfelt speech at the reception.
The timing of Ryou and Ichigo getting together varies greatly in fanfiction. It could be as soon as the beginning or might not happen until the climax. It all depends on the author's imagination and the plot they want to create.
I think Stefan can be a really liked character in these fanfictions even though he's in a bit of a love triangle situation. His complex relationship with Damon and his past with Elena make him an interesting addition. He might be shown as still caring for Elena but also coming to terms with her love for Damon. Also, Alaric is a character that many fans like. He can act as a mentor or a voice of reason in the fanfiction, and his relationship with Damon and Elena can be explored in different ways.