In terms of gacha, nutaku has been following the dmm gacha in perfect order.
http://神姫プロジェクト.攻略wiki.com/index.php?ガチャ
The next set for the upcoming revival event should be summer seasonal version of already existing girls but with different element (Fire Sol, Light Brynhildr and Light Nergal). They are time limited and will be gone after it ends. Unless nutaku decides to change it up for us, which is unlikely.