Imagine if they’d had flashbacks to the kind of things that made Thrawn intimidating, by showing the TIE Defender program in live action or his use of forced labour, through how people like Ryder remembering him. They could have done something.
They didn’t need flashbacks though. All they needed to do was to establish Thrawn as a viable threat. He could have built weapons factories, a new fleet - shown (the audience unfamiliar with the character) he is a resourceful & ingenious leader.
Much of the series was set in a new galaxy. Filoni had a blank slate to do anything. For example, this galaxy could be the home of advanced but pre-hyperspace civilisation. Thrawn, despite his depleted resources, could have used his superior knowledge & technology to conquer a system and convert its technologies into a military industrial complex from which he could rebuild his forces. Ezra could have been liberated by the local rebellion & become one of their leaders. That would also have demonstrated his worth to an audience unfamiliar with his character.
But instead we got a planet as dull & barren as Filoni’s own imagination. One where Sabine - on her own - is easily able to track down Ezra, making Thrawn look like an even more useless lump.