The (future) High Septon is not a bad guess at all. Gatiss could be awesome in that part played against Cirsei.

It would mean they'll play up the emergence of the Sparrows in s4, and perhaps, a bit like they did for Theon, they will show a bit the rise of the future High Septon, culminating in the assassination of the present one and the election of the High Sparrow toward the end of the season or early in s5.

It could be a clever thing to do... the High Sparrow character has to get ready to become Cirsei's central antagonist in s5. He has to replace a whole bunch of familiar characters involved with Cirsei in KL: Tyrion, Tywin, Jaime, Sansa etc. It wouldn't be a bad thing to flesh out and introduce the character earlier in the TV context., especially since they've not gone much into the church of the Sevens so far, and the power of the High Septon might come out a little out of nowhere (it could be sensed more in the books, where many allusions were made).

I'll be curious to see how far into the season the wedding occurs (we could get clues from ep. titles eventually). It's still very much up in the air if it will happen almost as the season opener with Tyrion's escape set for the mid-season big show, or if they'll keep it for the mid-season, with Tyrion 's escape happening only in episode 9 (with or without a reveal he's on a ship to cross the Narrow Sea in the season finale). In the first case they'll really need to set up the religious stuff early in s4 to get ready for the Margaery-Cirsei conflict that will begin at the end of the season. In the second case they might restrain it to the emergence of the Sparrows only, as what happens in KL after Tywin dies will not really start before season 5.

I can't decide if I'd prefer Gatiss as B. Rivers or as the High Sparrow but I guess either part would fit him much better than Mace Tyrell's.