Multi-world project view
With multi-world support on the way, and I think that a useful addition to the editor would be a multi-world project view, where all the worlds can be seen side-to-side in world space. This could really help with designing games with a large play area of interconnected worlds, by letting the user visualise how they connect in world space.
This could work very similarly to the world view. You would be able to move a world around in world space, which will still follow the movement rules set by its world layout. Moving the world would affect the world space positions of its levels. Clicking on a world would let you configure the settings of that world. Double clicking on a world would open that world in the world view.
To represent each world in the editor, all of a world's levels could grouped together and treated as one unit. To make it easier to tell one world from another and know what levels belong to what world, the editor could assign each world a configurable color and tint the levels. This would make a view like this a lot more readable.
Some games may not care about creating a consistent world space shared by all worlds, treating every world like it lives in its own disconnected world space. These games wouldn't stand to benefit from having this view at all. It makes sense to me then that this view would be optional, and it could be disabled in the project settings.