I am a huge fantasy nerd and I can't get enough of it. I read The Hobbit in the 7th grade and got hooked into Tolkien's world eve since. I like high fantasy more though.
To your question, no it won't be possible. There is no way they can get the cast back together. They will be older naturally and it won't just feel the same with someone else as Jon Snow or Tyrion.
GRRM's books are all we are going to have for a satisfying and drawn out ending.
Let's hope the spin offs have the same quality (as GOT season 1-6).
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