In "How Do I Make Him Drive" I discussed a few options, however since then another has revealed itself. If you watch the Star Wars Celebration: Anaheim BB-8 roll-out video closely, you may notice that he pretty much rolls around the same axis the entire time. That's interesting. There's another way to make a ball roll.
What I think they did (and pointed out by the BB-8 Builder's Club) is to have a "driveshaft" that runs between two points on the sphere. That makes him run in a straight line. To turn, there's a flywheel, likely hanging below the driveshaft as low as possible. Conservation of Angular Momentum means that when the flywheel is spun rapidly in one direction, the sphere has to rotate in the other direction. Together the movement is pretty natural.
|BB-8 on Stage at Star Wars Celebration Anaheim|
Furthermore, folks had notice an interesting ring on the model where it looks like there's a round access or construction mark. That ring surrounds the 1-2-3 triangle. Not sure if it's related to the driveshaft, but seems likely that there's something interesting to cause it.
|Ring on Celebration BB-8 Stage Model|
That technique solves a lot of concerns about my build. The interior was intended to be smooth to allow a "hamster" to roam, but with a driveshaft it doesn't have to be as particular. In addition, it allows for simplified wiring and placement of the LEDs... Eventually maybe even room for some of the toys in the hatch panels?
Anyway, that's what I'll investigate in the future.