@NecroKnight The way I'm doing this is that everything will proceed normally with them. The differences that will make changes, large or small, is the addition of characters. Battles that might have been very close might have different outcomes. More advisors might cause different canon character decisions to be made. I'm sorry if this is a deal breaker, but I'm pretty hard line on making your own character. In your interactions with canon characters you can still treat them as NPCs. If you look in the interest check, my second post contains this.
Also, as to the why not question. I am strongly of the opinion that you and I or anyone who didn't make the character will write them wrong. I'm not a fan of when comic books change writers or when shows change writers. I notice subtle differences in speech patterns and it irks me. I don't really like keeping canon characters as NPCs but I can bear that. Again, sorry if this ruins your ideas about the game.
Well it kind of does.
Considering that in Robert' Rebellion - most of the action and leadership WAS done by canon characters. There isn't much to do in my opinion, if we can't play any canon characters. Considering it's a time of war - and most of the leaders of the armies WERE canons in that regard.
One can't just replace away Robert or Renly or the Mad King. This is Westeros we are talking about after all - where the fantasy tropes are broken easily and no one man can change fate itself.