One possible plot could be that May and Drew have been rivals for a long time. As they compete in various Pokemon battles and contests, they gradually develop feelings for each other. A moment of victory or a shared adventure leads to their kiss. It could be that they are in a beautiful location, like a flower - filled meadow after a particularly intense battle, and the emotions just overflow, resulting in that kiss.
Maybe in the fanfic, there's a threat to the Pokemon world. May and Drew have to work together closely, closer than they ever have before. During this time, they discover new sides of each other. Drew might see May's determination and kindness in a new light, and May could admire Drew's skills and his hidden soft side. This new - found respect and attraction culminates in a kiss as they realize they don't want to lose each other.
The plot could involve a Pokemon - themed festival. May and Drew both participate in different events. They keep running into each other throughout the day. There are misunderstandings at first, like Drew thinking May is there just to show off, and May thinking Drew is being his usual arrogant self. But as the day progresses, they clear up the misunderstandings and end up sharing a sweet kiss under the colorful lights of the festival.