Amazon would be nice, however I doubt it will happen since Amazon's model so closely resembles Apple's. They wouldn't want to put something that directly competes with the iTunes store on the Apple TV.
As for HBOGo there might be more than just Apple and HBO agreeing to putting it on there. Not all cable/satellite companies support it on all devices. For example, when I had DirecTV, they wouldn't allow me to use HBOGo on my Roku box, so I had to watch it on my nosy Xbox 360. That's one of the reasons I dropped DirecTV.