We know about the `pale dragon` Tyrion had sighted, that may not have been Drogon, or may have been Drogon through a mist or cloud cover (who knows?)
When I finished reading the book, though, there were two other potential Dragons that left an impact on my memory as I was moving through the story for the first time.
The first was Euron and his alleged egg. The first book alludes to the value and rarity of Dragon eggs over and over. It seems ridiculous that Euron would throw one off the side of his ship into the ocean where it would be impossible to recover, especially when, apparently, "one egg is enough to buy an army."
If he actually did, there's a possibility Patchface told us it hatched:
"Under the sea, smoke rises in bubbles, and flames burn green and blue and black. I know, I know, oh, oh, oh."
But that said, it seems strange that he would just throw it off the side of his boat. However, I say that because I understand their value as a reader who gets Daenarys POV's. Maybe he did.
Maybe he didn't.
Maybe he traded it to the House of Black and White for Balon Greyjoy's assassination, which could tie into why Jacen was at the Citadel, possibly trying to steal rare books on Dragons, or something.
Whether he did or did not, I don't consider George RR Martin to be the kind of author who just throws mud at the wall. Even brief mention of petty Houses often seems very deliberate. Shouldn't we consider Euron just mentioning he had a Dragon egg (which would be the first we've ever heard of in existence outside Dany's) also deliberate?
It was a very specific action he had Euron admit to, and it comes out of seemingly nowhere. Euron has no real cause to just tell people that he had a Dragon egg. It was one sentence, and never mentioned again, and yet it stood out to me as significant.
I think he either
(a.) Threw it over the side and it hatched, and Patchface was referring to some terrible sea-dragon Euron's carelessness will unleash on the seas in the final books.
(b.) Kept it.
(c.) Gave it to the House of Black and White for Balon's tumble over a bridge, which is tied into `the Alchemist` at the Citadel.
Next it's the Ice Dragon.
It's implied that the magical powers of the Warlocks waned when the Dragons went out of the world, and arose when they returned out of Drogo's funeral pyre. I thought this connection between magical strength and Dragons was significance.
The books have been referring the Ice Dragon as a thing of lost legend numerous times, through numerous books.
If the Warlocks did indeed feel greater magical power in the presence of Daenarys' Dragons, than something Melisandre says in A Dance With Dragons suddenly becomes potentially very significant.
Her revealing to Jon that she had found her magical powers heightened and strengthened at the Wall for reasons she could not understand.
Ice Dragon encased within?
The presence of Dragons is the only thing we have seen in ASOIAF canon that strengthens magic, and Melisandre doesn't know why she is more powerful at the wall, where George is repeatedly telling us about Ice Dragons of old in POV chapters. Surely that revelation was a significant one for future chapters. I sincerely doubt Melisandre just happens to be `more powerful` at the Wall and that's just the end of it, including, but not limited to, a unique resurrection. The `Why`, however, is something I would fully expect to be answered, unless it already was, in Qarth