A fanfiction could revolve around a swimsuit competition in the Pokemon world. Lillie enters the competition, not really expecting to win but just to have a new experience. Along the way, she meets other trainers who teach her different swimming and posing techniques. In the end, she discovers a new side of herself and makes new friends.
One idea could be a story where Lillie overcomes her shyness about wearing a swimsuit. Maybe she's at the beach with her Pokemon friends and they encourage her to have fun in the water. At first, she's hesitant, but as she sees how much her Pokemon are enjoying the beach, she gains the confidence to join them.
Perhaps a story where Lillie's swimsuit has a special meaning. It could be a gift from a friend or a family member. While she's wearing it at a pool party, she gets separated from her group. Her Pokemon help her find her way back, and during this adventure, she realizes the importance of the swimsuit as a symbol of the bond she has with those who gave it to her.