While there's no written-in-stone way to do it, I recommend talking about what you're doing or planning to do in the game. Let the people know what you're thinking. And if you're doing a commentary talk about the WHOLE game, not just gameplay.
Talk about the music,
talk about the graphics and backgrounds,
talk about the systems and mechanics,
talk about the plot (if you like it or if you think it's dumb),
talk about things you've heard about this game,
talk about the developer,
talk about launch problems or dlc.
ANYTHING YA GOT, about the game.