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.
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, '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.
Dr. Eggman could be a great main antagonist. He could be using the chaos of war to further his own plans for world domination. He might be creating powerful war machines and using them to take over different regions, forcing Sonic and his friends to fight back to protect the peace.
A new character could be introduced as the main antagonist. For example, a being from another dimension who has been observing both Sonic's and Megaman's worlds. This being realizes that by combining the chaos energy from Sonic's world and the advanced robotics technology from Megaman's world, it can create a new, all - powerful empire. So it starts to manipulate events in both worlds to bring about this combination, while Sonic and Megaman have to stop it.
Dr. Eggman and Dr. Wily could team up. Dr. Eggman has his mechanical creations and knowledge of Sonic's world, while Dr. Wily is an expert in creating powerful robots in Mega Man's world. Together, they could be a formidable force against Sonic and Mega Man.
Dr. Eggman would be a likely choice as he often causes trouble for Sonic. In the context of Secret Wars, he could have found a way to connect Sonic's world to other dimensions and start the wars to finally defeat Sonic once and for all. He might be using new and powerful machines he has created from the combined technology of different dimensions.
Dr. Eggman would be an obvious choice. Since Sonic is now in a more vulnerable human form, Eggman could see this as the perfect opportunity to finally defeat Sonic and take over the world. He might come up with new devious plans, like creating robots specifically designed to target a human Sonic.
Another possibility is a corrupted version of one of Sonic's friends. Maybe through some dark influence, like an evil Chaos Emerald, a friend turns on Sonic and blinds him. This would add an extra layer of drama as Sonic has to deal not only with his blindness but also the betrayal of someone he trusted.
Perhaps a group of dark side cultists could be the main antagonists. They idolize the power of the dark side and see Ahsoka as a prime candidate to join their ranks. When she refuses at first, they start to hunt her down, but in the process, they unknowingly push her closer to the dark side, creating a complex dynamic where they are both the pursuers and the inadvertent cause of her fall to the dark side.