They could explore ancient ruins. Maybe they find a hidden cave filled with rare Pokémon. Ash's battle skills and Lillie's knowledge of Pokémon lore would come in handy.
Perhaps they travel to a far - off island. There, they discover a Pokémon that is said to have the power to control the weather. Ash is eager to battle it, but Lillie warns him to be careful as it could be very dangerous. They then have to work together to figure out a way to approach the Pokémon safely, maybe by befriending it first with the help of some of their own Pokémon.
One possible adventure could be them getting involved in a local Pokémon festival. They might participate in contests or help organize events. Ash could show off his battle skills in friendly exhibitions, while Lillie could share her knowledge about Pokémon care with the festival - goers.
One possible adventure is exploring the Kanto region's famous Pokémon gyms. Ash already has experience with these gyms, and he could guide May through the challenges. They might also encounter some rare Pokémon along the way, which would be exciting for both of them. Maybe they'll find a Pokémon that neither of them has seen before and have to figure out how to catch it together.
They could have Pokémon battles against powerful trainers in different regions. Ash's battle experience combined with Cynthia's strategic skills would make them an unbeatable team.
Their relationship could develop through shared experiences. As they travel, they rely on each other more. Ash helps Lillie become more confident in dealing with Pokémon battles, and Lillie helps Ash understand the unique aspects of Alola's Pokémon.
One possibility is that Ash's adventures would change. He might not be able to go on long journeys as easily with a baby at home. So he could focus more on local adventures or teaching his child about Pokémon.
Perhaps they will explore the hidden areas of Kanto. There could be rare Pokémon or mysterious items waiting to be discovered. They might also meet some old friends along the way and reminisce about their past adventures.
One possible plotline could be an adventure where Lillie and Ash team up to explore a new and mysterious region in the Pokémon world. They might encounter rare Pokémon and have to work together to overcome various challenges. Another could be a story about them training Pokémon together, sharing their unique training methods and learning from each other.
They could team up to rescue a kidnapped ninja. The cat could track the kidnapper's scent, and Naruto could engage in combat when they find the enemy. Naruto's determination combined with the cat's stealth would make them a formidable duo in this rescue mission.
They could go on a journey to find a hidden vampire lair. Maybe they'd encounter other mythical creatures along the way and have to use their wits to outsmart them.
They might have an adventure in an ancient, abandoned city. Ash is curious about the place, and Latias guides him through the crumbling structures. Along the way, they encounter some old - time Pokemon guardians that they have to convince to let them pass.