A possible plot is that a new person enters their lives. This person initially seems to be a threat to their relationship, but in fact, it makes them realize how much they truly care for each other. They then have to decide if they want to pursue a romantic relationship or keep things as they are.
An unexpected event can also be a catalyst for their relationship development. Such as getting lost on an island adventure. During this time, they rely on each other for survival, and in the process, they discover new aspects of each other that they find attractive. This could lead to the realization of their love for one another.
One idea could be that Stitch gets transported to another dimension where he meets characters from a different animated universe. They could team up to fight a common enemy that threatens both their worlds. Another plot could involve Lilo somehow getting superpowers and joining Stitch on an intergalactic adventure to save other planets. Maybe they encounter a race of beings who are trying to steal the unique bond between Lilo and Stitch to use for their own evil purposes.
Perhaps the pregnancy is a result of some sort of alien experiment gone wrong. Lilo and Stitch then have to find a way to reverse it if they don't think they're ready for a baby. But as time goes on, they start to warm up to the idea and decide to keep the baby, facing all the difficulties and joys that come with it.
One idea could be about Stitch learning new Hawaiian traditions with Lilo and her family. For example, they could be preparing for a big luau together, and Stitch has to master a particular dance or cooking technique.
One characteristic could be a more mature exploration of their relationship. Since it's about older adults, it might delve deeper into their life experiences and how those shape their interactions. For example, they could have more complex backstories involving past relationships or career achievements.
One idea could be a role - reversal plot. Instead of the usual power struggles between Kaguya and Miyuki, have them switch personalities for a day. It would be hilarious to see Kaguya being overly confident and forward, while Miyuki is shy and hesitant.
One idea could be that the two fated lovers are from different worlds, like one is a human and the other is a supernatural being. They meet by chance in a strange situation and start to feel an unexplainable pull towards each other.
One idea could be exploring the backstories of the characters in more depth. Maybe a character has a past trauma related to love that makes them act the way they do in the original story. Another could be a 'what if' scenario, like what if two characters who were enemies in the original fell in love in the fanfic.
Well, perhaps there could be a fanfiction where the Grand Councilwoman from Lilo and Stitch discovers the Pokemon world. She decides to send Stitch there on a mission to study the Pokemon. Stitch then forms a team with some Pokemon like Charmander and Squirtle, and they have to face off against an evil organization that wants to use Pokemon for bad things. It would be a fun mix of the two universes.
Finding lilo nani older adult fanfiction can be a bit tricky. Some fanfiction communities on social media platforms might be a good place to start. For example, on Tumblr, there could be users who share or write such fanfictions. However, the availability really depends on the popularity of this particular concept within the fan community.