@Gowi You are right, we have played a game together before. It was a superhero one. It was a bit ago though, and I was unsure if I was remembering correctly. Yeah GRRM will probably build up to it a lot better, but the actual specifics of her death are never elaborated upon in our backstory, it was just stated to happen. The same way I think the Boltons and Walder Frey will probably die differently in the books, but it's still likely going to happen.
Dredigan put our whole view of the matter in way better words than I did but you did bring up some good points about player involvement in the shaping of the world. For me it was kind of a balancing act to keep the canon (both of them) as relevant as possible while still having room for collaboration.
Now the reasons for me wanting to keep Shireen's death in the story are because of it's thematic appropriateness to the canon story as a whole, and because of the potential ripple effects. Since we have played a game before, I am not worried about your ability to play a Shireen character really well. But I do want to be fair to future player, and I don't think there's a fair way to allow certain character's deaths to be washed from the backstory and bar others. As of right now, I think I would prefer to keep Shireen's death in the RP's backstory. As Dredigan said, nobody wants you to play a character you don't want to play so if this is a hard no-go for you than I understand if you want to leave completely and there will be no hard feelings from us.
However, a good case can be made that a resurrected Shireen would be a very compelling PC for the game for the reasons already stated by Dredigan. It would depend on the specific circumstances of her resurrection, if she came back soon or in adulthood, but either way it could potentially enrich her POV and the story greatly. Of course, the role of the Baratheon bastards would be adjusted depending on whether or not you wanted to have them as additional PCs as well.
I hope you do decide to stick around. But whatever you decide to do, I'm glad we had this discussion and that it's been a sensible and reasoned discussion.