Well, they could start to be more open about their feelings. They might hold hands more often, have secret glances at each other during meetings. Their relationship could become the talk of the team, with some members being really happy for them and others being a bit jealous.
A plot could involve Allura being injured. Lance is the one who stays by her side and nurses her back to health. During this time, they have deep conversations and get to know each other on a more personal level. When Allura finally recovers, she is so grateful and realizes her feelings for Lance have changed. She initiates a kiss, which is a turning point in their relationship and also cements their bond as a couple within the team.
A lot of the fanfictions about their kiss also explore the reactions of the other characters in the Voltron team. Some might be supportive, while others could be initially shocked or confused. The kiss can also be a symbol of a new era in the team's dynamics, as Keith and Lance's relationship changes. It can lead to interesting storylines about how the team adapts to this new development.
One common theme is the exploration of forbidden love. Since there might be some circumstances in the original story that make their relationship a bit difficult, this often comes up in fanfiction. For example, they might be from different groups or have some sort of rivalry initially.
The popularity of Keith and Lance kiss fanfiction might be due to the strong chemistry between the two characters in the original source. Their personalities are quite different yet complementary, which makes the idea of them getting closer very appealing.
You can try Archive of Our Own (AO3). It has a vast collection of fanfictions including ones about Lance and Keith. Just search for the relevant tags and you should be able to find many stories with their kiss scenes.
One popular one could be 'The Unexpected Bond'. It beautifully portrays the relationship between Lance and Keith leading up to their kiss. The story builds on their shared adventures and how they gradually realize their feelings for each other.
One common theme is the overcoming of internal barriers. Both Keith and Lance have their own insecurities and issues that they need to deal with before getting to the point of a kiss. Another theme is the exploration of their different personalities coming together. Keith's more brooding nature and Lance's more outgoing personality can create interesting dynamics in the lead - up to the kiss.
One popular one might be 'A Moment of Truth' where their kiss is a culmination of a long - built - up tension during a battle - scarred journey. It beautifully captures their emotions.
There's also 'Love's Encounter' where the story focuses on their relationship development leading up to that special kiss. The author does a wonderful job of showing their connection through various events and dialogues. Kitty and Lance's characters are well - portrayed, and the kiss scene is quite intense.