An organization that wants to control or eliminate any form of magic or special abilities could be the antagonist. Since in this fanfiction, if Will has certain powers and Gideon is involved with him, the organization would target them both to maintain their idea of a 'normal' world.
One idea could be that in this reverse situation, Will is the one with a mysterious power that Gideon is trying to understand. For example, Will might be able to see into other dimensions, and Gideon, being the curious character, is drawn to him to figure out how it all works.
The Green Goblin could be the main antagonist. He could use the concept of 'Uno Reverse' to his advantage. Maybe he creates a device that reverses Peter's powers, as I mentioned before. With Peter weakened and confused, the Green Goblin could launch a full - scale attack on the city, knowing that Spider - Man is in no state to stop him.
One idea could be that they start off as rivals but then find themselves in a situation where they have to team up to solve a mystery in Gravity Falls. As they work together, they begin to understand each other better and a strange bond forms between them.
In Reverse Falls Gideon x Dipper fanfiction, it's interesting because it takes two well - known characters from the original Gravity Falls and flips them in a way. The setting of Reverse Falls itself is already very different and full of mystery. Gideon's and Dipper's relationship can be explored in ways that are not possible in the normal Gravity Falls setting. For example, they could be involved in more morally ambiguous situations or have a more antagonistic - yet strangely attracted - relationship. It gives authors a lot of room to play with their personalities and create unique plotlines.
One popular Reverse Falls Gideon x Dipper fanfiction is 'The Darker Side of Gravity'. It delves deep into their complex relationship in the Reverse Falls universe. Another good one might be 'Gideon's New Plan' which shows an interesting take on how their relationship could develop in that strange and spooky setting.
You can focus on creating unique characterizations. For example, give Reverse Dipper some new quirks or abilities that are different from the regular Dipper. For Gideon, you could explore a more complex backstory that affects his relationship with Reverse Dipper. Also, use vivid descriptions of the Gravity Falls setting to make the story more immersive.
One option could be Kaguya from the Naruto universe. Her god - like powers would pose a great threat to Bayonetta. She could be trying to expand her influence into Bayonetta's world, seeing it as a new place to conquer.
It could be a dark wizard who wants to capture the nymph for her powers. He sees the nymph's connection to nature and wants to use it to gain ultimate power in the Naruto world.
It could be a rogue ninja who has also discovered a way to harness the power of Amaterasu in a corrupt way. This ninja might want to use the power to take over the ninja villages and Naruto stands in his way.
It could be a new character who has the ability to disrupt both Lambo's time - traveling powers and the Reborn characters' mafia - based powers. This antagonist might be after a powerful artifact that Lambo and the Reborn crew are protecting.