Looks like they are casting Mace Tyrell, Oberyn Martell, and Elaria Sand:
One can only hope Kevan Lannister returns as well. He was in his minor role in Season 1. I'm sure everyone has forgotten him already. But, with two of his sons killed by Rickard Karstark this season (should have been only one with the other in a prisoner exchange) and Lancel's storyline over the past two seasons, he's pretty necessary for the Small Council next season. A Small Council of only Twyin, Tyrion, and Varys at this point is pretty silly.
Cool to have Ellaria, although her part will be small I presume. The Sand Snakes for season 5 will be condensed. Obara, Tyene, Nymeria will likely become one character. Sallera is needed separately. And, the young ones can be condensed into nameless children or eliminated altogether I would expect. They love killing off siblings - see the two Tyrell sons.