One plot idea could be a story where Tyki Mikk and Allen Walker are forced to team up against a common enemy greater than their differences. Maybe a new, extremely powerful Akuma that threatens to destroy both the Order and the Noah's side. This would put them in a situation where they have to rely on each other's unique abilities.
A plot could also involve a time - travel element. Allen Walker somehow travels back in time and meets a younger Tyki Mikk. Allen tries to change Tyki's future, to prevent him from becoming a Noah. This would lead to a complex web of interactions as Allen tries to influence Tyki's life without disrupting the time - space continuum too much.
The character dynamics are often complex. Tyki Mikk, as a Noah, represents chaos and the side against the Order that Allen Walker is part of. But they can also find common ground in their shared humanity. For example, they might both enjoy simple pleasures like a good meal or a beautiful sunset, which can create a momentary truce between them.
One great Tyki Mikk x OC fanfic is 'A Meeting of Souls'. It delves deep into the relationship between Tyki and the original character. The author does an amazing job of creating a believable backstory for the OC that ties in well with Tyki's own story in the original work. It has some really sweet moments of interaction between them.
A crossover with the world of Fullmetal Alchemist would also be cool. Allen could meet the Elric brothers. His Exorcist abilities might be seen as a form of alchemy by some in that world. They could all work together to uncover a conspiracy that involves both the Homunculi and the Millennium Earl.
First, you need to understand Tyki Mikk's character well. Read the original material to get a sense of his personality, his motives, and his backstory. Then, create an OC that can complement or contrast with him in an interesting way. For example, if Tyki is mysterious, the OC could be very straightforward.
One plot idea could be that the chibi Allen Walker discovers a hidden power within himself while on a simple mission. Maybe he stumbles upon a mysterious artifact that unlocks a new form of his Innocence. Another idea is that he gets transported to a parallel world where everything is in miniature like him, but there are different rules and he has to figure out how to get back. And also, a plot where he befriends a group of other chibi exorcists and they form a special team to take on a powerful Akuma that no one else could defeat.
One plot idea could be that the adopted Barry Allen discovers he has a unique connection to a parallel universe where his birth family resides. He then embarks on a journey to find out more about his origins while still maintaining his relationships in his adopted family. Another could be that he uses his speed - force powers in a creative way to solve problems within his adopted family, like preventing a family member from making a big mistake in time. A third idea is that an enemy targets him specifically because of his adopted status, thinking it makes him vulnerable, but Barry proves them wrong.
Some fanfictions may focus on Allen Walker's relationship with a particular vampire. For example, they could start as enemies but gradually develop a deep bond as they understand each other's motives and the difficult situations they are in.
One interesting Allen Walker fanfiction could be about his further adventures in a world where the Earl has been defeated but new threats emerge. It might explore his relationships with the other exorcists in a more in - depth way, like how he and Lavi become even closer friends and partners in maintaining the new - found peace.
A plot could revolve around Caitlin and Barry being forced to work undercover as a couple to infiltrate a criminal organization. As they pretend to be in love, they start to realize their true feelings for each other. Or perhaps Caitlin's scientific knowledge and Barry's speed are the only things that can stop a city - wide disaster caused by a malfunctioning S.T.A.R. Labs experiment.
The fanfic might explore a romantic relationship between Allen and Road. In this story, they could start off as enemies but gradually discover their feelings for each other through various interactions and shared experiences. For example, they might be forced to work together on a mission that makes them see the good in each other.