One common theme could be the idea of an unlikely friendship. Percy, being from his own world, and Bulbasaur from the Pokémon world, would make for an interesting friendship dynamic. Another theme might be adventure, as they could go on quests together in different settings.
One common theme could be the bond between Ash and Bulbasaur. Since in the Pokémon world, they have a trainer - Pokémon relationship, in fanfiction, it might be explored in more depth, especially with the 'lemon' aspect possibly adding a layer of emotional or even romantic undertones.
I'm not sure of specific 'percy x bulbasaur fanfiction' off the top of my head. But you could try searching on fanfiction websites like Archive of Our Own or FanFiction.net. They usually have a wide variety of fan - created stories, and you might be able to find some interesting ones about this pairing there.
Friendship is a very common theme. These three characters are often depicted as having a strong bond that helps them through various difficulties. For example, in many fics, they rely on each other during battles or when facing internal demons.
A common theme is redemption. Many fanfictions explore the idea of Gaea being redeemed through her interaction with Percy. Since Percy is often seen as a symbol of hope and goodness in the original series, his influence on Gaea can be a way to show that even the most malevolent beings can change.
The theme of cooperation is also prevalent. They usually face some sort of challenge or enemy that they can only overcome by working together. This shows how two very different individuals can achieve great things when they put their differences aside and collaborate.
One common theme is the clash of two different worlds. Percy, from his heroic and somewhat mortal - associated world, and the titaness from a more ancient and powerful realm. Another theme is often about redemption or change, where the titaness might be on a path to change her ways because of Percy.
One common theme could be friendship. Even if they start off as rivals or indifferent to each other, the story might develop their relationship into a strong friendship. Another theme could be self - discovery. As they interact, they might learn more about themselves.
The theme of loyalty is also common. They have to be loyal to their own kind while also developing feelings for each other. For example, Percy is loyal to his fellow demigods, and Amphitrite has her duties as a goddess. How they balance this loyalty with their new - found relationship is often explored in these fanfictions.
One common theme is adventure. Percy often goes on adventures with the goddesses, facing various monsters and challenges. Another is love or attraction, as there are often romantic undertones between Percy and the goddesses.
Friendship is a common theme. Since it involves Percy and multiple characters, how they form friendships and support each other is often explored. For example, Percy might help a character in need, and a strong bond of friendship is created.