As a creator of Star Wars’ new canon, you don’t go and add a character like
Grand Admiral Thrawn to the canon lightly. It’s a character fans have been craving, but he also comes with a lot of questions. Where has he been during the Empire’s rise? Where does he go when the original trilogy begins? In an interview during Star Wars Celebration Europe, showrunner Dave Filoni promised us he has the answers.
Source: io9 – Star Wars: Rebels Showrunner Talks Thrawn, Ezra, and the Series’ Endgame