There's enormous potential for an Obi-Wan movie (or movies). It's unrealistic he stayed in his hut for 20 years and did nothing, especially with him being a foundational member of the nascent rebellion. As others have mentioned, we know from the films that Obi has a somewhat active history on Tatooine: he has familiarity with Mos Eisley (a spaceport, what's he doing there if he's not going anywhere?), he has a contentious relationship with Uncle Owen, and he has some experience with Tusken Raiders. There could be a little or a lot there for a movie depending on how you approach it.
Most importantly, there are two outstanding items I think make for potential stories. Darth Maul is one, as has been mentioned; with his resurrection being canon and his story unresolved at the end of CW, there's some possibility there.
Most of all, Vader. At the end of ROTS, Obi-Wan leaves Anakin for dead. He has no reason to believe he survived. In ANH, Obi-Wan knows who Vader is. How? If he sat in his hut for 20 years and did nothing, there's no way he could know. Clearly they had a subsequent encounter between films that explains this. How you navigate that and maintain Luke's secrecy is the story. Obviously, Obi-Wan leaves Tatooine for some reason at some point, and in the course of that runs into Vader.
I loved Ewan McGregor as Obi-Wan. Definitely the best thing about the PT. I'd love to see him again.