Yuzusoft just released Natsuzora Kanata's OP! You can check it out on
Youtube if you want to, as they also uploaded it there.
Seems like my hunch was right. Haruka Shimotsuki and Sakakibara Yui, who did the ED and the OP of ExE respectively, are also doing Natsuzora Kanata's, though this time it's Haruka Shimotsuki with the OP and Sakakibara Yui with the ED.