There has been no official word on the plot of the movie, however, based on references to '54 and the original film and the tone and all, when the early images of the design leaked, many fans, myself included, speculated that this could be a resurrected '54 Godzilla that has changed due to how he was reborn, etc.
Bob Eggleton posted an interesting theory around the odd tip of his tail that I think could hold some truth in the end, that it is a regenerating '54 Godzilla and since he was disentegrated by the oxygen destroyed, he's rebuilding by absorbing material from marine life. Shots from yesterday show the tip of the tail to clearly be comprised of many skeleton looking formations, it's like his whole body is made up of recylced material.
