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.
At the moment, there's no definite information on whether Berserk manga will continue. It might, but it could also remain unfinished. A lot depends on the circumstances and decisions of those involved in its production.
Yes, it is. Berserk has a rich and engaging plot, along with well-developed characters. The world-building is also top-notch, making it a standout manga.