Being honest, I hope Fox McCloud and Pikmin on the movie are just some small easter eggs/cameos, like, if you want do make a movie of these franchises, do it, do not put everyone on the same thing.
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About R.O.B., he's an accessory, and for me his apparition is equivalent to, for example, the references to Hanafuda, and he has appeared on Mario Kart DS before. Also, I don't mind small references like the ones to Kid Icarus, Balloon Fight and Ice Climber on the first movie.
But putting a character from a completely different franchise to be a major character is kind of weird.
Well, I don't remember seeing Fox in the 2007 hit Wii video game Super Mario Galaxy.