It is meant mostly for 2D maps.
It could work similar to Tiled (
http://www.mapeditor.org/) but without the tile part.
You would have a map with a specific size (in pixels, not tiles) in which one could insert back- and foreground images (with settings like "repeated" or "auto-movement" for x or y) and layers.
Layers include objects like blocks, text, particle effects, starting points etc. These are defined by giving them an unique name, width and height and assigning them an image.
One could simply place instances of those objects in the selected layer. There should be an improved version of the "insert next to" function of the Secret Maryo Chronicles Level Editor.
And of course one should be able to save and load these maps as XML, JSON or Whatever
TM.
It's not like i need such an editor at the moment, but i have been playing around with Swing lately and am thinking about starting programming such an editor for practise. So i want to know, because i don't feel like reinventing the wheel. I found already a .Net-based editor, but that is of course Windows only.
qubodup (i believe) once said, that level editors should be included in the game. I think he is right in general, but except Frogatto most 2D-games i know about have level editors which are somewhat clumsy or downright awful to use.
Partial issues are:
* no or too hidden help
* strange key shortcuts
* not enough powerful features
* no mouse support (the keyboard isn't really good for selecting objects with a selection rectangle or positioning stuff)
To push this to extremes that could mean for such an editor:
* rotating support
* more like an IDE with the ability to manage whole projects
* managing and creating image to id mappings
* plugin support for custom import and export formats
* plugin support for scripting facilities like a graphical DSL or a source code editor with lua support
* the shiny green play button to start the level in the test mode of a game executive like game -t "path/to/level.xml"
(Not that I'm insane enough to plan these, but that's my ideal for such an editor)
EDIT: Since this post may not answer you question properly: the basic idea is an editor where you can place objects where you want, not bound to tiles.