I'm sorry, but a stand-alone Yoda film is not a good idea. I'm not saying that a good Yoda movie can't be done—talent can often overpower a bad idea. But exploring Yoda's backstory is a dead-end that only weakens the character. It hurts the original films. It renders the creative soil of the Star Wars universe fallow.
The modern blockbuster film industry reminds me of the comic book industry in the 1990s: Obsessed with gimmickry and dead-set on milking any popular intellectual property to the point of irrelevance. Now, the "mainstream" comic industry only appeals to a few thousand diehard fans who want to read the same stories over and over again. And comics are propped up by a film industry that's mining those stories for the first time in a series of well-received films. Who's going to prop up the film industry when all those ideas are mined? Where are the new ideas going to come from? After you've revealed all the mysteries behind a 33-year-old supporting character, where else is there to go?