My personal vote, is Kingdom Hearts HD Remix. Maybe not the immediate next series, but sometime soon. He's already expressed an interest in the most recent Full Play episode, so it would be cool to see that.
I'd be up for any kind of RPG. My personal choice would be Final Fantasy VII as it is my favorite game and I'm pretty sure it's one of Michael's as well. However, I'd be fine with any sort of RPG, be it final fantasy, dragon quest, zelda, or some of the zillions of others both Japan and American.