This is just my opinion, but I don't believe in saying something will never happen. This is Hollywood, if something makes the slightest profit studios will make more. I remember years ago people use to say we will never get a Spiderman movie, we will never get an Avengers movie, we will never get another US Godzilla movie. Well guess what? We have plenty of Spiderman movies, plenty of Marvel movies (included two Avengers), and two more US Godzilla movies to look forward to.
Personally I don't care for a US Mothra movie, but if the next two Godzilla sequels, Skull Island and Pacific Rim 2 become hits (which I believe they will), then spinoffs will become a strong possibility. Giant monster movies are making a comeback, and if they continue making profit then studios will be making more and more. Just my opinion
