Since I don't have direct access to 'bickslow lemon fanfiction', it's hard to say exactly. But generally in fanfiction, common themes could be character development, relationships with other characters from the original work, and exploration of the character's backstory. However, if the 'lemon' aspect implies something inappropriate, then that's not something that should be promoted or detailed.
I'm not familiar with the specific 'bickslow lemon fanfiction' themes. But in fanfiction in general, themes often revolve around love, friendship, and adventure. For a character - specific fanfiction like one about Bickslow, it might also include his unique powers and how they interact with the story. But again, if 'lemon' has an improper connotation, we should avoid focusing on such content.
Well, without having read 'bickslow lemon fanfiction', we can only speculate. If it's in line with proper fanfiction, it could potentially have themes related to Bickslow's growth as a character. Maybe it shows his interactions with other characters in new and interesting ways. But if the 'lemon' part refers to adult or inappropriate content, then the real focus should be on positive, creative, and legal themes that can be found in regular fanfiction about the character.