I too have wondered about the movie maker tool, though I do not think we can really assume much about what it was going to be, maybe it could be something you can put on youtube, or maybe something else?
While Kovu is not here to show us what it might have become, we can atleast imagine what it could have been like.
I would like to believe it could be something where you can combine the all of the other 'makers' and use to create a movie, y'know, selecting a map, a weather, placing down objects, and then even characters, which you can set to do an action a certain time.
Just brainstorming there x3 Well anyway, while we may never know what that button could of held, due to Kovu's absence, we may, at least, dream.