I don't think this movie is needed, but really none of the "Star Wars Story" movies proposed are at this point. Rogue One wasn't a necessary story but still made for a cool war movie spin on Star Wars. Han Solo isn't but could still be a cool space western. Same thing with Obi-Wan really. I think they just want to give McGregor another shot with a writer and director who aren't George Lucas.
There are possibilities with this. I don't know what they've done in the new-canon comics. I know that after ROTS, priority one for Vader was tracking down Obi-Wan so that could be the focus behind another galaxy-spanning adventure. Problem is Vader and Obi-Wan can't see each other again until New Hope. The more likely possibility is a smaller-scale story localized to Tatooine and an overall more personal story for Obi-Wan.
I'm thinking a film where Obi feels kinda hopeless, sees the Empire sweeping through the galaxy so easy, guilty he never finished Anakin off. Gets wrapped up in something in Mos Eisley, goes on some wild adventure where as one of the last jedi in an occupied world, he is a fucking force of nature. Comes back feeling more optimistic and ready to wait for Luke to be ready for the saber.
Yeah I just want a badass wreck shit Kenobi.
This is pretty much what I had in mind -- do a Tatooine western. Make it about the despair of being one of the last Jedi. The central plot itself shouldn't even have much to do with the overall Star Wars story, just an excuse for Kenobi sort out his own inner demons. Bring back a couple aged Clone Wars characters. Those stories of "old Ben" Luke grew up hearing had to come from somewhere.
Probably put some makeup on McGregor and grey out his hair.
Wonder if we'll get a Liam Neeson force ghost.
This too. Doesn't even have to be full force ghost. Just Neeson's voice for a couple lines would be great. Even if it's just important flashbacks from his training or something.