Well, a plot could revolve around Kanata and Miyu getting separated in different dimensions while in their UFO baby forms. Each of them has to face unique challenges in those dimensions. Kanata might end up in a dimension where time moves differently, and he has to figure out how to get back to Miyu. Meanwhile, Miyu could be in a dimension full of strange creatures that she has to befriend or outwit to find a way back to Kanata and their normal world.
One interesting plot idea could be that Kanata and Miyu discover a new power related to their UFO baby forms. For example, they might be able to communicate with an ancient alien race that left behind clues on Earth. Another idea is that they have to deal with a new villain who is also after the secrets of their UFO baby origins. This villain could be causing chaos in their town, and Kanata and Miyu have to use their unique abilities to stop him.
A great plot idea could be that Kanata and Miyu are invited to an intergalactic gathering of UFO babies. At this gathering, they meet other beings from different planets who have similar powers but also different cultures. They have to navigate through the social and political aspects of this gathering. There could be some misunderstandings due to cultural differences, and they end up having to solve a big problem together with their new friends to save the intergalactic community of UFO babies.
In the fanfiction, their relationship could develop further by sharing their dreams and fears. Maybe one night, under the stars, they start opening up about what they are really afraid of and what they hope for in the future. This emotional vulnerability would create a stronger bond between them. Also, if they start to teach each other new skills, like Kanata teaching Miyu how to better control her UFO baby powers and vice versa, it would also enhance their relationship.
I haven't actually read this specific fanfiction about miyu and kanata kissing. But generally, such a fanfiction might explore the relationship between the two characters in a romantic way. It could show their deepening feelings through this kiss, perhaps as a turning point in their relationship.
Often, miyu is written as someone who is very in tune with her emotions. This can lead to some really interesting character interactions in fanfics, especially when paired with kanata. Kanata may be seen as a bit more of a mystery, and fanfic authors might explore his backstory and what makes him tick in relation to miyu. For example, in some fanfics, his past experiences could be what shapes his relationship with miyu, whether it's through shared traumas or simply different outlooks on life that they gradually come to understand about each other.
Another theme could be growth. Through this kiss, miyu and kanata might experience personal growth as they become more open and vulnerable in their relationship. They might start to see each other in a new light and grow as individuals as well as a couple.
One idea could be that a group of teenagers discovers an abandoned UFO in the woods. They start to explore it and accidentally activate some kind of alien technology that gives them superpowers. Another could be a world where UFOS are common and there is a special agency that deals with the relations between humans and aliens. A third idea is that an alien from a crashed UFO befriends a lonely human and they go on adventures together.
Rather than focusing on lemon fanfics, you might explore regular miyu and kanata fanfics that focus on their relationship development, adventures they go on together, or their character growth. These can be found by using the search functions on relevant fanfic platforms and filtering for the appropriate tags.
One idea could be that baby Mario and baby Luigi get lost in a magical forest filled with strange creatures. They have to use their baby wits and little jumps to get through obstacles and find their way back home. Maybe they befriend a little fairy who guides them part of the way.
One idea could be that Baby Mario and Baby Peach go on a magical adventure through the Mushroom Kingdom, encountering new and strange creatures along the way. They might have to work together to solve puzzles to reach a special power - up that can save the kingdom from a new threat.
One idea could be a story where the baby in the 'baby project' has some sort of special power that the characters have to figure out how to manage. Maybe the baby can communicate telepathically and it leads to all sorts of comical and heartwarming situations as the characters try to understand what the baby wants.
One plot idea could be that Baby 5 and Sai are on a mission together. They start off not getting along well because of their different personalities. Baby 5 is impulsive and Sai is more reserved. But as the mission progresses, they begin to understand each other's strengths and develop a mutual respect. They end up saving each other during a dangerous encounter and form a strong friendship.