"I'm considering grabbing a slice of the beginning of my visual novel and putting it as a demo, so the answers would help me know what to do."
Please do this. I'd love to see what Harmful Heaven has to offer in demo form!
Personally, I think a "hook" approach works when the player is presented early on with a good impression, and are thus left wondering what will come next, without it always being a cliffhanger. If the basics of a project are presented clearly and interestingly, a player/reader will end up asking themselves how these basics will be expanded upon in the future. Charming characters, engaging mechanics, and intriguing writing/dialogue are good ways to do this as well, but maybe that's obvious.