I think I agree with some of the thoughts shared above.
Game of Thrones (book 1) gave us the perspectives of the Stark family (Ned, Catelyn, Arya, Sansa, Bran, Jon), Tyrion and Dany. Book 2, there was no Ned but we started to add peripheral characters... Theon, Davos. Book 3 gives us Jaime and Brienne, kills off Catelyn, etc.
But the way I see it, the characters that have been keys, since the very beginning, were... the surviving Starks, Jon Snow, Tyrion and Dany.
I wouldn't expect Aegon, or Euron Greyjoy, or Ramsay Snow, or anyone who wasn't introduced until Book 4 or 5 to be the final champion of Westeros. If GRRM didn't feel the need to write them into the first three books, when it was originally intended as a trilogy, then I doubt they'll be the last ones standing at the end of the seventh book.