Yes, there are many interesting Sonic Dark Gaia fanfictions. Some explore the relationship between Sonic and the Dark Gaia in more depth, like how Sonic might have to deal with the lingering effects of the Dark Gaia's power. Others might create new adventures for Sonic in a world still influenced by the Dark Gaia.
Well, 'naruto dark gaia fanfiction' could be about a darker take on the Naruto universe with elements related to Gaia. Maybe it involves Naruto facing some sort of dark power related to Gaia, like dark entities or a corrupted form of nature energy. It could also explore how Naruto and his friends deal with this new, darker threat in their world.
Well, 'dark gaia monster sonic fanfiction' could be about Sonic facing off against a monster related to Dark Gaia. It might involve Sonic having to use his speed and wits to overcome the challenges this Dark Gaia - inspired monster presents. Maybe it's set in a world where Dark Gaia's influence has created this menacing creature and Sonic is the only one who can stop it from causing chaos.
A great storyline might be that Sonic discovers a way to communicate with Dark Gaia on a mental level. Instead of just fighting, he tries to reason with it. He finds out that Dark Gaia is in pain and its destructive actions are a result of that pain. Sonic then has to find a way to heal Dark Gaia rather than destroy it. This would be a very different take on the typical Sonic vs Dark Gaia scenario.
Well, obviously Dark Gaia itself could be the main antagonist. It has the power to cause chaos and destruction, and Sonic would have to stop it from wreaking havoc on the world.
Sonic is most likely the main character. Since it's a Sonic fanfiction, he would be the one to take on the Dark Gaia monster. His friends like Tails or Knuckles could also be in it. Tails might be there to provide technical support, while Knuckles could help in combat or with knowledge about the Dark Gaia entity.
Gaia from 'Gaia tells her story' could be anyone really. She might be a woman from a particular community or culture. Maybe she is an artist or a scientist who wants to share her professional and personal journey. Without more context, it's difficult to define her precisely.
I think Shadow could be an interesting addition. His brooding nature and his own set of powers would contrast well with Sonic's more upbeat personality. He could also have his own reasons for getting involved in the Sonic - Dark Gaia conflict. Maybe he has some past connection to Dark Gaia that no one else knows about. Also, Amy. Amy's love for Sonic can drive her to do some really brave things in the face of Dark Gaia's threat, which is always an interesting dynamic.
You can try some popular fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'naruto dark gaia' on their search bars, and you should be able to find relevant fanfictions.
One idea could be Sonic having to face a corrupted form of Dark Gaia that has new and unexpected powers. Maybe it can manipulate time in certain areas, causing Sonic to have to race against time in a whole new way.