Legendary (the studio behind these movies) flat out owns King Kong, while Godzilla, Gamera, Ghidora, etc have to be licensed from Tojo in japan. They probably wanna focus more on King Kong for a movie or two before bringing Godzilla and company back again, I can see the next movie being completely in Hollow Earth with new monsters from there.
It would be wild if they showed 20 million years ago or whatever when Kong's ancestors made the throne and fought Gojira's grandpappy, it would be great because there probably wouldn't be any humans but I could see studios really hesitant to do anything like that. They'll probably do a mix, like a 20 min flashback or history sequence before modern day Hollow Earth.
Nothing is announced so far, although I expect they are working out plans on what to do next since this movie is clearly making money.