I think the open source community can definitely do something cool like this. Although good UX has been historically the Achilles Heel of open source, I believe the focus would help a lot in this regard. No need to worry about the engine part itself. Godot's editor for example is already quite good, and it's only a portion of the engine. Imagine what a team with more focus could achieve.

It would also help tremendously in standardising file formats and other conventions. We've kinda settled on either ECS or Node-Based architectures, so there's an obvious starting point, concept-wise. Just have each engine provide a configuration file telling which components (or node types) are available, and how they can be configured. Or provide a language-server-like API so it's extensible.

There's no need to worry about high-fidelity, physics, sounds, or scripting during the editing (only during debugging), so some editor that just knows the 3D formats is enough. If debugging is needed there should be an optional way, like with code editors.

I've been looking for a big project to tackle, maybe that's something I can try to start myself :)