Obi Wan,Boba Fett Or Yoda as the Third stand alone film?
Personally I would go Obi Wan over them both. As I would see the Boba Fett film being boring and not feeling a Star Wars film. Yoda as cool as it would be to would also be silly basically be a muppets film following him around. But Obi Wan could follow him through 3 different parts of his life in between the films.
As a youngling/Padawan being trained by Qui Gon before Episode I. Then training Anakin before Episode II. Then as a old man watching over Luke before Episode IV so much to fill. And Ewan Mcgregor would still be perfect to they can digitall make him younger and older in the film.