I guess not everyone read the stuff from GD UK 2013? It is Lorgar... that was confirmed in September that he is the next Primarch release. Horus is confirmed as Primarch #5 (Following Lorgar), Mortarion is confirmed as Primarch #7 (For obvious reasons). I'm assuming the Sculptor from Lorgar is already working on Primarch #6. Simon Eagan stated that he is working on Horus and a preview picture was shown @ GD UK.