I was reading through your ideas for a WORLD OF ICE AND FIRE video game set during the “Age of Seven Crowns” and it struck me that the Great Southern War begun by King Gyles the Third might make a perfect skeleton upon which to build a bigger story (for we know enough to pique our interest, yet enough is left obscure for creativity to be uninhibited) : may I please ask if you think this seems reasonable?

Depends on the genre. 

That might work for a Hero-style RTS, where you can tell a story around the tactical combat, small group management, resource management, etc. 

I don’t think it works as well for an RPG, because you generally want the player to feel like their character/party is the protagonist of the story who the plot is going to revolve around. 

But if you want to do an RPG during a war, you could do worse than the way Witcher 3 did it, where you’re operating on the fringes essentially as a secret agent or dealing with issues like the Wild Hunt which are orthogonal to the conflict between Radovid and Emryr.