[@Gowi] Well the nuts and bolts of the situation is largely subject to collaboration between the players. What is Jaime Lannister's fate? Brienne of Tarth's? Jorah Mormont? Who did Brandon Stark marry? Arya? Sansa? What's the status of House Frey? Is House Bolton bouncing back? Who are the other members of the Small Council? What other big rivalries and conflicts are boiling over? All other kinds of specific details. Details like that are important but are ambiguous and undecided in what is essentially my high concept summary of the potential future. Notice I pretty much gloss over the particulars of nearly 25 years of history in Westeros to allow room for that kind of collaboration. But big elements somewhat have to be nailed down since the RP is both set in the future and is also a blend of Show/Book canon, and something of my attempt to merge elements of the two continuities. Shireen was then envisioned as dead since the RP draws from both continuities. 

I thought it would be best to lay down a solid foundation for the players to then build on. I didn't really think it'd be great to leave open the question of who would sit on the Iron Throne and big details like that. In my opinion, leaving so much up in the air at the start would have really delayed the start of the game and possibly killed it as people debate about whether or not Jon, Dany, Aegon, Stannis, Tommen, or whoever should sit on the Iron Throne. I also thought quibbling over what elements from the show should and shouldn't be canon would get really tedious. Especially since in most cases like Shireen, they will almost certainly die in the books as well. Creating a high concept summary to try and reconcile the two continuities seemed neater. My GMing style in the past has mostly been for me to help build something of a sandbox and then let the players decide the direction and details of the story and world. That's just my view on it but I understand completely what you're saying. 

That said, I'm still open to hearing your ideas for your Shireen concept. And yes if she is alive, she'd be in her mid-thirties at this point. 

EDIT: Dredigan suggested to me perhaps that since she was sacrificed to R'hllor, it isn't completely far-fetched that the magic of R'hllor was also used to bring her back in the "modern day" of the RP. It could potentially add an interesting facet to her character and some very interesting opportunities for interaction or conflict in the game. What are your thoughts?