I have to say I'm not surprised he's joined the cast, no matter the role. Just seems like it makes a ton of sense to have him on the show somewhere.
But they haven't said who he is yet. What are some possiblities? I saw one of the Greyjoys mentioned. I can't really see that, Gatiss to me seems too bumbling for a Greyjoy. Maybe he will be a Tyrell?
He doesn't have the right physique for Mace (it's not terribly important except Mace's weight and premature aging, beside his lack of political acumen, is kind of relevant to the fact it's his mother and children at the forefront and not him), but why would they keep it "secret" if he's cast as "the father of Margaery and Loras"? Nothing special to hide there... the wedding is not a spoiler and the character has been mentioned by his mother last season.
Gatiss is at his best playing either hilarious or very bizarre/disturbing characters or a mix of both (he's first a comic actor, though often in the weird/dark vein). Even his Mycroft was a bit like that.
Hard to see who would fit his profile in s4, but since they're far into Bran's storyline already, perhaps the three-eyed raven? This character is a big spoiler and it would justify the producers keeping the part Gatiss has been cast for secret in case one of the mass media elaborates too much on the character's role in their announcement if they gave them a name to go look for in the books.
I guess he could eventually play one of the Greyjoys, though again it's hard to see why they'd wish to keep his part secret then.
