Great theories everyone! I’d also like to add that the real reason why Rumpel brought magic into the world is because he knows that the lines between each reality are fading and that impending war is certain. So, in an effort to defend this reality, he brought magic into the world so that Storybrooke would have a line of defense against all the different worlds. Because when you really think about it, why would Rumpel want more power? We saw in “The Return” that he was ready to give up his powers and lust for magic. He gave August the dagger to show him just how sincere his intentions were. So, why would he all of a sudden do a complete 180 and decide to go after more power? It dosn’t make any sense. And I also don’t think that Rumpel needs magic to find his son. Unless Baelfire is trapped in some mystical tomb burried miles under the Earth, than magic is not needed for this task.
So, with those theories discredited, what’s left? I think Lil’Red and Midnight Dreary are correct in their assessment that the worlds are probably slowly dissolving and seeping into one another. And if that’s the case, then magic is going to be needed in order to defend Storybrooke. The complete dissolution of all worlds and dimensions is definitely Apocalyptic.
I also believe that the 20 doors inside of Jefferson’s hat will be the main players in the final battle. Those will be the worlds who’ll “take the battlefield”