To create an engaging ui for a visual novel, you need to consider the target audience. If it's mainly for younger players, you might want a more colorful and cartoonish ui. For an older, more mature audience, a more sophisticated design could work. Also, make sure the ui is intuitive. The player should be able to quickly figure out how to access different parts of the story, like the menu or the save/load options. You can also use sound effects related to the ui actions, such as a click sound when a button is pressed, to make it more immersive.
One way to make an engaging ui for a visual novel is to tie it in with the story's theme. For instance, if the visual novel is set in a medieval world, you could use a ui design with elements like parchment - like textures for the menus and old - fashioned fonts. Incorporate relevant imagery into the ui as well. Another important aspect is to ensure good contrast in the ui design. This helps the text and interactive elements stand out clearly, especially for long reading sessions. And don't forget to test the ui on different devices and screen sizes to make sure it looks and functions well everywhere.