I think that the Cassian series has the most potential usefulness out of every announced upcoming SW project. It's supposed to be set 5 years prior to RO/ANH, so it'll deal with the only aspect of the Lucas saga which he never really delved into: the rise of the Rebellion. Everything else is more superfluous in terms of revolving around characters whose key moments in the saga were already taken care of in the films.
If
Andor is able to give the Rebel cause more depth, with a real sense for their fledgling struggle, it could actually even enhance the OT. Making Leia's involvement more poignant, and Luke joining all the more of a turning point. None of the other shows have that potential, IMO (quite the opposite, in fact). Plus, it could offer a distinctly different tone in the SW landscape (even more than RO) while also providing the writers plenty of thematic material to deliver to the audience. Y'know... why stories should be told in the first place?
Starting something from nothing; opposing authoritarianism with principled purpose; navigating shades of gray with espionage, subterfuge, and gritty struggles... chances to show cool **** happening, introduce new characters on both sides, tell compelling stories, and provide actual themes!
With all that said, Filoni will probably just hijack it to make another live-action version of Rebels. And also do **** like use an entire episode to explain how/why Leia knew that Obi-Wan used the name "Ben" Kenobi when Luke called him that during the detention rescue.