In my opinion, Skull Island doesn't seem to be a prequel to any of the existing movies (Kong '33, '05 etc.), but likely just a prequel to the Kong legend. So if that's the case, this could be the beginning of a new world for Kong, meaning the events leading to New York can be re-imagined in anyway Legendary sees fit.
So, when this movie comes out and we see Kong a lot bigger than usual, then in my opinion we are definitely getting a Kong Vs Godzilla, or a crossover movie with Kong, Godzilla and other monsters.
I cannot fathom that Legendary isn't exploring the possibility of a crossover between these two iconic characters. Whatever legal hurtle they need to jump over with the studios, I think Legendary will make it happen if they believe this will be a huge hit. And I think it will.
In Hollywood, I don't believe the word "never" exists. Anything is possible. Years ago I remember people saying we we would never get an Avengers movie. Well we got one, with a sequel on the way and a Justice League movie on the horizon! And let's not forget a Batman Vs Superman movie.
I remember on this very board people would say we will never get another American Godzilla movie. We got one, with at least two sequels on the way. So will we get a movie with both King Kong and Godzilla? I say most definitely YES!