IMO "GUIs built around something declarative and event-driven" covers systems like Houdini and Massive - they're "declarative" in the sense that the scene graph

IMO "GUIs built around something declarative and event-driven" covers systems like Houdini and Massive - they're "declarative" in the sense that the scene graph is defined beforehand, and "event-driven" in that they react to external inputs with everything from behaviors in Massive to demand-driven lazy calculation in Katana.