It must be character driven and able to go dark. Obi-Wan would be at or near a personal crisis point and over the course of the film must deal with his demons in order to become the wiser Ben we all adore.
Whether or not the entire film would take place on Tatooine is debatable. There could be an excursion to find other Jedi survivors and/or reveal some nefarious Imperial plot that Obi-wan could secretly smash without revealing his identity. Preferably on some totally dark world like Nar Shaddaa which would contrast perfectly.

It should be very reminiscent of the better stories such as the Kenobi novel, and would completely rely on the lead actor to sink into the role.

Will Disney really go for it? Heck no. They’re too busy making another film yet again about the Death Star.