Captain Rex appears wearing Rebel Commando outfit, confirming the fan-theory that he is the white-bearded Rebel Soldier seen on Endor in Star Wars: Episode VI - Return of the Jedi (1983) formerly identified as 'Nik Sant'. This is the character who briefly disguises himself in Scout Trooper armor before being captured during the battle of Endor.
Thrawn's fleet includes ships based on Imperial Dreadnoughts that featured prominently in his trilogy of books by Timothy Zahn which introduced Grand Admiral Thrawn.
An earlier version of the script included a scene collecting various Imperial officers of note that included Director Krennic from Rogue One: A Star Wars Story (2016). His animation model was designed, but not built as the scene was cut early in production.
The original title for "A Fool's Hope" was "Ezra's Gambit".