>>30693838UBs are definitely gonna be in the movies, that's where the "serious" stuff happens. We're also definitely getting Nihilego in the main series because it's connected to the plot. But since it seems likely at least one of the box legendaries, if not both, will also show up in the series for the same reason, Magearna has already had a movie and Marshadow hasn't been officially unveiled yet, the UBs are basically all they can do for the films.
