User blog comment:ImGianChan/Cover for Volume 8/@comment-3003168-20120816011846

Unless it's the final volume, I think it's safe to assume it will be Saeko. I don't think the current rotation (Rei, Saeko, Saya) will change until the final volume, which I think will include all three. There was an alternate cover for Volume 4 with Shizuka and Alice, but I don't think they will make it onto a standard cover.