You get to unlock Cassandra's adult form and the last unfortunate monster that was released in the Collab gets an alternative humanlike form that you can sex. Morrigan gets vaginal sex scene too if you wanted this too. No new characters releases, though in the gallery there's a Demon Queen Cassandra form that probably comes from Part 3. Oh, and lots of new jobs and races that you can unlock, plus you can give more races to other characters with Promestein.
The big cookie here is the timeloop event, which halfway I ended up noticing something that ended up being right. It's another collab event with one of the games they have collabed with, kinda. It's a long event because you have to figure out how to break the loop by contacting with different characters and asking the right question, and it has lots of new scenes that you can only watch from that event (though you can access them from the gallery later if you want to watch them again).
After beating the event you have access to the festival in which you can go with either Alice/Ilias or Sonya in a date there. Which is nothing spectacular because you're just checking what other characters are doing in it with the character you selected giving dialogue to everything. Torotoro plans to add other characters in the future, though I doubt they are taking into account the polls because I was watching them for a while and I started to see some bottom characters suddenly rise to the top in a very unnatural way like Succubus Witch.
Oh, and to unlock the gallery go to Luka's Inn and check the beds at the furthest right. One of the options unlocks everything.