One common theme could be the exploration of their relationship dynamics. It might involve how they interact, whether it's in a friendly way or with some underlying tension.
I'm not sure about specific 'once ler x greed ler fanfic' off the top of my head. You could try searching on popular fanfic websites like Archive of Our Own. They usually have a wide variety of fanfictions in different categories.
In Greed Ler x Reader fanfictions, a prevalent theme is the exploration of emotions. Since it's a relationship - based fanfic, the development of feelings between Greed Ler and the reader is a big part. There are often themes of trust - building as well. The reader may start off being cautious around Greed Ler, and over time, trust is either built or broken. Additionally, the theme of mystery is common. Greed Ler may have secrets, and the fic revolves around the reader uncovering those secrets. And of course, the theme of attraction, whether it's physical or emotional, is almost always present.
Finding Greed Ler x Reader fanfiction can be quite an adventure. One place to look is Wattpad. While it has a wide range of content, you need to be a bit more selective. Some authors on Wattpad write really engaging Greed Ler x Reader stories. Additionally, Reddit can be a useful resource. There are subreddits dedicated to fanfiction where you can ask for recommendations specifically for Greed Ler x Reader fanfics. You could also try searching on specialized fanfiction search engines, which might have more targeted results for this particular pairing.
Fanfics basically involve fans creating their own narratives based on the elements of something they love. It can be anything from imagining new adventures for characters to exploring alternate storylines. They're a way for fans to express their creativity and passion for the source material.
It's hard to say they are widely popular. They are likely only popular within very specific, niche communities that are interested in the particular theme related to a cockring. Most mainstream audiences would not be into this type of fanfic.
First, you need a clear idea or concept for your fanfic. Then, decide on the characters and their development. Start writing with an engaging beginning and build a compelling plot.