One possible plot could be that Shun and Alice are on a dangerous mission together. During a tense moment, they realize their feelings for each other and share a kiss for good luck or as a sign of their new - found closeness. Then they go on to successfully complete the mission with their bond strengthened.
In a more emotional plot, Shun could be going through some personal turmoil. Alice is the only one who really understands him. They spend a lot of time together, sharing their thoughts and feelings. One day, while they are in a beautiful and quiet place, maybe a garden in Vestal, Alice reaches out to comfort Shun. Their eyes meet and they slowly lean in for a kiss. After that, they have to deal with the reactions of their friends and the implications of their new relationship within the Bakugan world.
Perhaps in a fanfiction, there is a Bakugan tournament. Shun and Alice are competing against each other at first. But as the tournament progresses, they start to see each other in a different light. They have to team up against a common and powerful enemy. In the heat of the battle, they accidentally get close and share a kiss. This kiss then becomes a turning point in their relationship and they end up working together more closely to win the tournament.