1. Listing of Entities
In a sidebar or in a widget, we need to list entities in a way that makes sense to the user. I was thinking that a "Recently Created/Added" section which would show the 3 to 5 newest entities, plus another section with "All Entities" would do the trick. All sections could be expanded/reduced, and entities would be grouped by tool in the "All Entities" section.
See also: version 2 of the entity picking mockup.
2. Selecting an Entity
When hovering a listed entity, a border could show up, and the background color could change. The arrow would also change to a hand.
3. Dragging an Entity
When an entity is selected to be dragged in the workspace, a cursor in the workspace would show the destination on the entity.
4. Dropping an Entity in the Workspace
When dropping an entity, an options screen would pop up, asking for more information on the behavior of the entity. Depending on the type of entity, there could be different settings, including display options, format options, and conditions. Lat but not least, there should be a check box to save a default behavior for a specific type of entity, allowing the user to drag and drop other ones of the same type without seeing this screen.
5. Visual Aspect of Added Entity
Depending on the chosen settings, an entity could look like that. By default, clicking anywhere on the entity would trigger the behavior, but there could be exceptions.
6. Hovering an Entity in Edit Mode
In "Edit" mode, an entity could be deleted or edited through a menu that would show up when hovering it, but there could also be the same kind of visual as in Nathan's widget designs (with a "Menu" button, an "X", etc.). I based this one on Flickr.com.