Hm if that's the case, it should be easy. I'm going to post the enemy concepts that was given to me, but I was thinking that due to the fact we shouldn't have original characters, I thought about how to recycle the cast. For example:
Meiling a lot of show time, such as in the entrance and garden boss
Patchouli and Koakuma is in the library
Dungeon for Flandre
Remilia for the Dining Room, her room, then the clock tower.
Then for the stages it'll be,
The gate entrance -> You can't get in the front door due to the barrier even after defeating Meiling.
Find an alternate entrance by going through the garden, just as soon as you found out a way, Meiling comes in again and stops you. You can wander about the mansion, and you can either go in the dining room first and face remilia but can't proceed on for a while as you will need a key then fight Koakuma and Patchouli in the library and get thrown to the dungeon or the vice versa.
After this, should we include puzzles? What achievements should we have? One thing I could think of is having a complete the map achievement by roaming around the mansion and unlocking special rooms.
For weapon upgrades, I was thinking you get different kinds of knives and the ultimate weapon would be some magically enchanced knife or something that breaks after the Remilia final boss battle or something!