One idea could be that Ash and Misty visit Unova for a Pokemon tournament. They meet new friends and face new challenges together. Misty could use her water - type Pokemon in creative ways against the Unova Pokemon's unique types.
Perhaps in the fanfic, Ash and Misty are invited to Unova to help with a problem. There's a strange phenomenon affecting the Pokemon in a certain area. They team up with some local trainers and use their combined knowledge and skills. Ash's experience from his previous travels and Misty's expertise with water - type Pokemon could be crucial in solving the mystery.
A fanfic could be centered around Ash and Misty getting lost in Unova. They have to rely on each other and their Pokemon to find their way. Along the way, they discover rare Pokemon and learn more about Unova's culture. For example, they might visit a local village where they are taught about a special Unova - based Pokemon training method.