Palpatine's journey into becoming the Emperor has been well documented in various forms of media over the years (many of which are no longer canon), but it sounds like his descent into madness would have been the focus of Star Wars Underworld. The series had hundreds of scripts written for it back when George Lucas was in charge of things, but was never actually made due to the sheer expense.
During a recent interview with VultureBeat, God of War helmer Cory Barlog talked about working with Lucasfilm on an Underworld video game, and shed some light on what sounds like a great show.
"When I was working at Lucas, I was allowed to go up to the ranch and read the scripts for the [canceled live-action Star Wars] TV show," he explained when the site asked about his involvement. "It was the most mind-blowing thing I’d ever experienced. I cared about the Emperor. They made the Emperor a sympathetic figure who was wronged by this [frick]ing heartless woman. She’s this hardcore gangster, and she just totally destroyed him as a person. I almost cried while reading this. This is the Emperor, the lightning out of the fingers Emperor. That’s something magical. The writers who worked on that, guys from “The Shield” and “24,” these were excellent writers."
Disney are currently concentrating on their big screen Star Wars projects and have no plans to work on any small screen spinoffs. Regardless, it sounds like they may have some good material to work with if they go back to these Star Wars Underworld scripts and borrow some material from them.