I had a theory while standing around at work today (I get a lot of time to think at work)
What if Monster Hunter is, in fact, playable in this game, and the reason they showed off the Rathalos boss first was because you unlock characters by defeating bosses in Spirit Mode, and they're planning to detail how that works in the upcoming direct while they show off what Spirit Mode actually is?
This is, of course, assuming any leaks we've seen are false and we have no idea what the final roster is. this was certainly something I just came up with while bored. Cool idea though, if you ask me.