Good question! For one, there are actually game idea generators out there, so you can always try them out and see if you find anything interesting (just replace the setting with "Vampire" by default): https://letsmakeagame.net/game-idea-generator/
Second, try to find inspirations! Any other games you played that you particularly enjoyed, and mechanics you thought would be great to redo? You can open a Vampire book on a random page and find something interesting there, read it in the search for stories to inspire you, browse Vampire tag on itch and see if anything gives you a "I want this, but different" idea.
Good luck!