I love the replies, guys. Keep em coming!
I personally believe they can explain Ghidorah as many others have mentioned.. Either as an ancient alien asteroid or one that crashes onto Earth in the present day. The more I think about it, the latter scenario is more likely.. Think of that scene as the asteroid comes barreling toward Earth, and then a short glimpse of something rising out of the carnage. Thats your teaser trailer right there..
Mothra and Rodan could have a similar origin to Godzilla. But that is boring and harder to explain as is. Were Mothra and Rodan at the bottom of the sea, leaching radiation from the Earth's core? Of course not.
A more likely scenario for them is to think back to GINO (I know, its sacrilegious, but bear with me.) How was GINO born? By the nuclear bomb being dropped on top of an island containing iguanas. Well, at the end of Godzilla 14, did a massive nuclear bomb not detonate? Could some of the impact of that nuke going off potentially cause creatures to be mutated.
However they approach it, I am excited to see!