Unity 3D, the increasingly popular WYSIWYG—ish engine for building video games, does a lot of things really well. Helping you build in-game user interfaces? That’s not one of them. At least, not yet.

Unity, the cross-platform game engine that powers games like Rust, Hollow Knight, and Pokémon Go, has introduced a new, controversial fee for developers, set to take effect next year. Indie developers ...