Well, on one hand, if the fanfiction is written well and focuses on character development rather than the improper 'lemon' elements, it could be a great exploration of the Percy character in the context of the goddesses. It might show new sides of Percy's personality, like how he deals with the power and allure of the goddesses. However, the 'lemon' aspect overshadows any potential positive aspects as it goes against the general moral and ethical standards of literature. So overall, it's hard to say there are truly positive aspects when 'lemon' is involved.
The only possible positive aspect could be the creativity of the author in combining Percy with the goddesses. But since 'lemon' usually implies sexual or inappropriate content in fanfiction, this really taints the whole concept. In a more positive, non - 'lemon' fanfiction, we could see interesting storylines about Percy's relationships with the goddesses, like his loyalty, his courage in the face of their power, and how they interact on a more platonic or heroic level. But in this case, with the 'lemon' factor, it's very difficult to find real positive aspects.
If we ignore the 'lemon' part which may involve inappropriate content, it could be positive in terms of creativity. Fanfiction allows fans to explore different relationships and scenarios for characters like Percy and the goddesses.