Another idea is to set a fanfiction in a different time period, like in the past. How would Dark Shadow have behaved in a more primitive and dangerous world? Maybe it was more wild and less controllable, and Tokoyami's predecessors had a really hard time taming it.
A cool idea for a Dark Shadow fanfiction could be a story where Dark Shadow gets separated from Tokoyami and has to find its way back. During this journey, it discovers things about itself and the world that it never knew before.
The 'Plant Symbiosis' quirk would be a great addition to a fanfiction. The individual with this quirk can form a symbiotic relationship with plants. They can communicate with plants, make them grow rapidly, and even use plants as an extension of their body for things like creating shields or lashing out at enemies. This quirk could bring a whole new element of nature to the story.
Dark Shadow is a really interesting character. In fanfiction, we can explore its potential in different ways. For example, it could be given more complex emotions and motives. Maybe it has a hidden side that we haven't seen much in the original series, like having a fear of being alone which drives some of its actions.
One cool quirk idea could be 'Shadow Mimicry'. The user can blend into any shadow and move freely within the shadow realm, also being able to mimic the actions of those whose shadows they are in. It gives a lot of stealth and surprise attack possibilities in a fanfiction.
One idea could be that a kaiju suddenly appears in the My Hero Academia world and the heroes have to figure out a new way to combat it since their usual powers might not be as effective. For example, they might have to work together in a more strategic way than ever before, combining different quirks to create a new type of attack.
Another approach is to do a Halloween party at UA High. Everyone dresses up in different costumes. Deku could go as All Might in his younger days. There could be a costume contest, and some characters might use their quirks to enhance their costumes. But then, a real threat shows up disguised as a party guest, and the students have to use their wits and powers to stop the danger while still in their party get - ups.
One idea could be that Godzilla arrives in the My Hero Academia world during a major villain attack. The heroes initially see Godzilla as a threat but soon realize that he is targeting the same villains who are causing environmental destruction. His massive size and power could be a game - changer in the battle.
Perhaps a plot where Mineta enters a parallel dimension where the rules of his quirk are different. He has to adapt quickly to survive and find a way back home. This could lead to some interesting character development as he faces new challenges and has to rely on himself more. He might also make new friends or enemies in this parallel dimension.
Maybe the new home is a large treehouse - like structure built in a forest. It would blend in with the natural surroundings and provide a peaceful yet secure place for the heroes. They could have different levels for different functions, like a training area on the lower levels and sleeping quarters on the upper levels. It could also be a place where they can study and analyze the quirks of villains and come up with counter - strategies.
Another concept is that Todoroki was turned into a vampire by a villain's quirk gone wrong. He then has to find a way to reverse it. He teams up with Midoriya, who is determined to help his friend. They search for a mysterious old hero who might know a cure, all while Todoroki battles his inner vampire nature.
One idea could be that Kakashi is somehow transported to the My Hero Academia world. He could become a teacher at UA, using his ninja skills and wisdom to train the students in new and unique ways. His Sharingan would be a very interesting power in the MHA universe.