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.

The numbers are pretty staggering. Battle royale MMO juggernaut Fortnite has over 250 million registered users(Opens in a new window). And in a survey last year, 41 ...