I played a completely freeform rpg over an AOL chatroom back in the day.

I’d imagine most rules light games would work fairly well over Hangouts IM as you can roll dice in the chat box.

I do feel like part of the success of the AOL chat game I played is that we scheduled time to all sit at our computers and play. I imagine you’ll still have posting rate problems in IM unless there are some posting speed ground rules.