There could be a fanfiction where Tom Hiddleston plays a soldier in the same war as in The War Horse. His character might cross paths with the main characters from The War Horse. He could bring a different perspective to the story, perhaps with his own backstory of a noble but conflicted past. And through his interactions with the horse and the other characters, a new adventure unfolds.
One interesting storyline could be Tom Hiddleston's character being a soldier who forms a special bond with a war horse, much like in the original 'War Horse' story but with his own unique charm added to it. Maybe he's a soldier who is initially afraid of horses but through his experiences with this particular war horse, he overcomes his fear and they become inseparable on the battlefield.
One popular theme could be redemption. Tom Hiddleston's character might be a broken man who finds purpose and redemption through his interactions with the war horse and the people in its world. Another theme could be friendship. His character could form a deep friendship with the horse or the other characters in the story, much like the bonds shown in the original War Horse story.
Goodreads has some groups dedicated to fanfiction as well. Join relevant groups and ask for Tom Hiddleston War Horse fanfiction recommendations. Members are usually quite helpful and can point you in the direction of some great stories. Additionally, you could try searching on Reddit. There are subreddits related to fanfiction and specific to Tom Hiddleston where users might share or recommend some good examples of this type of fanfiction.
One great 'Avengers Tom Hiddleston Fanfiction' is 'Loki's Redemption'. It shows Loki in a new light, dealing with his past actions and trying to earn the trust of the Avengers. It has some really interesting character development for Loki.
One common theme is romance. Many fanfictions explore the relationship between the oc and Tom's character, often with a lot of passion and emotional depth.
One possible Tom Hiddleston love story could be about a fan who meets him at a movie premiere. She accidentally spills her drink on him. Instead of getting angry, he smiles and they start chatting. They find they have a shared love for classic literature. Over time, their friendship blossoms into love as they exchange letters filled with quotes from Shakespeare and other greats.
I don't have that specific knowledge. But you could try looking at entertainment news outlets or fan forums to find out if he's attending Comic Con 2018.