I thought about this recently and I'm not sure if it's a theory already or if it holds any weight--but I was wondering if there's any evidence/any theories of Daenerys and her army crossing the Narrow Sea after winter comes. Sort of like how the Vandals et al. crossed the Rhine into Rome after it froze over in winter or how Fingolfin crossed the frozen Helcaraxë into Middle Earth in the Silmarillion. I know George is all about mixing history with fantasy...
I don’t think I’ve heard of any such theories, sorry. (Which doesn’t mean they don’t exist, try googling to see if anyone else thinks the same?) And I don’t think there’s any evidence either – no foreshadowing, no dreams or prophecies (unlike Dany’s Trident dream), not even a historical record of the Narrow Sea ever freezing over. The only thing close was TWOIAF mentioning that the Rhoynar tell stories about the Long Night, saying that the Rhoyne river froze as south as the joining of the Selhoru (about the latitude of Oldtown, above the foot of Dorne). But that’s a freshwater river (at max a mile or two wide), not a huge body of salt water like the Narrow Sea. Approximately 250-300 miles wide of salt water, about 2000 miles long… if that’s freezing up, Dany had already better be in Westeros, because the end of the world will be very soon.
Also, an army won’t do a damn bit of good if it’s that frickin’ cold, especially not Dothraki or Unsullied from warm Essos. (Dragons, otoh…)
Mind you, because of Dany’s Trident dream, I have wondered if the Bite might freeze over, after the Wall falls. Just so the Others can cross from northern Westeros to the south. (Specifically the Others – wights can swim, alas, and also the dead can rise wherever the Others’ cold winds blow.) But the Bite’s a lot smaller than the Narrow Sea itself. And it’d be the Others crossing, confident and freezing everything around them as they go with their ice technology/magic, not scared-shitless too-damn-cold human beings. Many people theorize that the Others won’t get further than Winterfell, though… but you can imagine it, can’t you? The swagger, their strange icy language and mocking laughter… and the Others coming, relentless, all around them cold and dead and frozen and empty… brrr, I’ve got the shivers now, and it’s nearly July.