We can tell you, however, that sources say McShane has a relatively small amount of screen-time during the season, yet his character is of key importance.
Dayne was supposed to be like 25 years old when the whole thing went down. I suppose they could completely change that and make him an older guy though if they wanted. Mance was supposed to be like late 30s early 40s and they turned him into a senior citizen so who knows.
The Mad King Aerys in a flashback scene makes a hell of a lot of sense to me. It's quite likely season 6 is filled with flashback overload for a variety of reasons as the story now calls for it. He'd make about the best psychotic maniac I can think of and the whole "short screen time important role" thing fits.
He'd be great for either
Game of Thrones casts Ian McShane in season 6 mystery role - ( New Window )
The entire area of my fucking asshole is now one gigantic fucking throb. I have no idea what's transpiring in there.