Probably fans of the Titan series in general. Since it's a fanfic about a Titan character, those who are already interested in the Titan universe would be the most likely audience.
One interesting aspect could be the unique character interpretations. Maybe it presents Bob in a way that's different from the original source, giving him new traits or backstories. Another aspect might be the plot twists. It could take the basic concept of the Titan and create unexpected storylines.
Since I'm not familiar with specific Bob Barnard and Bob Armstrong fanfic, it's difficult to say. But common themes in fanfic in general can include friendship, adventure, and overcoming challenges.
Eren could be an interesting choice. Since he has such a complex character arc already, making him the tenth titan could add another layer to his story. He could struggle with the new power and how it changes his view of the world and his mission.
One possible fanfic could be about Mikasa's transformation into a Titan being a unique situation in the 'Attack on Titan' world. Maybe she has different abilities compared to other Titans. For example, she could retain more of her human consciousness and use her combat skills in Titan form. This would create an interesting dynamic as she tries to protect her friends while being a Titan, which is usually seen as an enemy.
One possible fanfic plot could be that Eren has been planning this for a long time. He might gather intelligence about the Colossal Titan's weaknesses. Then, in a fierce battle, he manages to get close enough to deliver a fatal blow. This could be a very emotional moment for him as well, considering the history between them.
Well, unfortunately, without more information about who Bob Barnard and Bob Armstrong are in the context of the fanfic (like if they're from a particular show, movie, or game), it's really hard to recommend specific fanfic. But generally, when looking for fanfic, you can also check out related fan communities on social media platforms. They might be able to point you in the right direction.
To develop Bob Armstrong's character, you could give him a backstory of having faced a great loss in the past, which has made him a bit reserved. As the story progresses, through his interactions with Bob Barnard, he could start to open up more. For example, if they are in a work - related fanfic, Armstrong could initially be hesitant to take risks, but seeing Barnard's boldness, he could gradually become more adventurous.
One possible plot idea could be that Bob Armstrong and Bob Barnard are two detectives working on a mysterious case. They start off with different approaches but gradually learn from each other and combine their skills to solve the crime. Maybe there's a shadowy figure behind a series of art thefts in the city, and they have to navigate through a web of false leads and dangerous encounters.
A great plot for such a fanfic could be that Mikasa's Titan form is a result of a connection to an ancient Titan power. As she becomes a Titan, she starts to have visions of the past, of how the Titans came to be. She then decides to use this knowledge and her new form to try and end the Titan - human conflict once and for all. Along the way, she has to face her friends who are initially scared of her but eventually come to accept her new form as she proves that she is still the same Mikasa deep down, just in a different body.