Hey I have a few ideas for you to take freely Indivi (note: I am in scrub/fanboy mode).
Firstly: Can you possibly add more monsters and creatures. Possibly drawing them from different mythologies such as Egyptian, Greek/Roman, Norse, Celtic, and or Slavic.
Secondly: Base a few characters and locations of stated mythologies.
Thirdly: I feel like your character should be able to be abducted by the watcher's but instead of being consumed by said entity like the succubuses state you can learn more spells from them. Like seductresses chain's, Emperers Cannon, Demention Warp, and etc.
Fourthly: Can you be allowed to re-enter Don's hideout after Cassie's story. Possible being a sex slave for a temporary amount of time.
Fifthly: Can you possibly add a hierarchy to monsters like grunt slime slime gang etc.
Sixthly: does anyone want to add on to what I have said for I do encourage all those to blossom and develop their creativity.