On the topic of sequel monsters, of course this movie is opening with the most popular ones. Ghidorah, Monthra and Rodan. With Kong coming next after these.
After that, there are still plenty to choose from. Anguirus, Gigan and Destroyer immediately come to mind as monsters people would love to see Hollywood's take on. Of course the list goes on. Kumongo and Biollante would be well received as well.
Down the road after the series is starting to get played out they could go off the deep end for Mechagodzilla and make my dreams come true.
However I wouldn't be upset if they went with original creations in the future, as long as they're as cool as the monsters they're passing on in favor of their original creations. Gotta be cool. Hollywood already recycles enough, I might actually prefer if they start introducing new monsters instead (after doing the top 3 in this movie).
It would also be nice to see a monster or two return from this movie in the future.
And let's not forget everyone's secret favorite:
Skeleturtle, the Mystery Bones of Infant Island!