One great 'admiral thrawn fanfiction' is 'Thrawn's Ascent' which delves deep into his early days in the Chiss Ascendancy. Another is 'Thrawn's Alliances' which explores his unexpected partnerships. 'The Thrawn Enigma' is also a must - read as it focuses on the mystery surrounding his character.
I think 'Thrawn's Hidden Agenda' has some great plot twists. You start off thinking one thing about his actions, but then it takes a completely different turn. Another one is 'Thrawn's Betrayal', where the betrayal is not what you initially expect.
Perhaps in the fanfic, Thrawn has a change of heart after interacting with certain Rebellion members. He realizes that the Empire's goals are not in line with his own sense of justice. He joins the Rebellion secretly at first, using his position in the Empire to gather intelligence and undermine Imperial operations from within, until he can fully defect.
One interesting fanfiction could be about Thrawn's early days in the Chiss Ascendancy, exploring how he developed his military strategies. Another might be a story where Thrawn has to team up with unlikely allies within the Star Wars universe to face a new and powerful threat. And there could also be a fanfic that delves into his relationship with other key characters like Ezra Bridger in more depth, perhaps showing their interactions during a different set of events than in the official canon.
A lot of fanfictions also explore Thrawn in crossovers with other Star Wars eras. For example, him interacting with characters from the Old Republic era or the time after the fall of the Empire. These crossovers can bring new perspectives to the Star Wars lore and show how Thrawn's skills and personality would play out in different historical contexts within the Star Wars universe.
One of the well - known authors is [Author Name]. Their fanfictions about Thrawn are really detailed, especially when it comes to his strategic thinking. Another good author could be [Another Author Name], who often writes stories that explore Thrawn's character from a psychological perspective. There's also [Third Author Name] who is great at creating exciting battle scenes involving Thrawn.
If Thrawn joined the Rebellion, it could change the way the Rebellion is perceived. His reputation as a fearsome Imperial might make some factions within the Rebellion hesitant, but his skills would eventually win them over. Also, his presence could lead to new alliances as other species and groups see the Rebellion as a more viable option with Thrawn on board.
You can start by looking on fanfiction platforms like Archive of Our Own (AO3). It has a vast collection of fanfictions, and you can simply search for 'Grand Admiral Thrawn crossover' in the search bar. Another option is FanFiction.net, which also has a lot of Star Wars - related fanfictions.
Eli Vanto is important. He is sort of an assistant to Thrawn and learns a great deal from him. Admiral Ar'alani also plays a role as she is from Thrawn's own people, the Chiss. And there are various Imperial officers who interact with Thrawn and either support or oppose him.