A widget engine should be considered as a standard part of the install.
Option 1 Just include in a pre-existing engine
Consider having gdesklets or some similar widget engine as part of e standard install. Maybe place a calender widget on the desktop by default.
Option 2 In the File Manager
A new take on Widgets. Instead of implementing them only on the desktop, or special widget panels, plug the widget engine into the File Manager.
Adding a calendar to the desktop would be as simple as copying the control file for the calendar into ~/Desktop (Drag the widget). Widgets that are less used can be placed into other folders. The control file would just be an XML file that indicates the widget to load, and saved settings.
Consider this scenario. I've calculated a grand total cost for a project using the calculator widget on my desktop. I drag a copy of the calculator into a shared folder containing all the relevant files for the project, and rename the calculator "Projected Total Cost". Other users entering the folder see the projected cost calculation right there.
Or consider this scenario. I have a folder for sharing files with a remote office in another time zone. I would drag a clock widget into the folder, and set the time zone for the clock to the time zone on the remote office. Right away when I go looking for new files in the folder, I see what the time is there.
Option 3 Make Widgets the Cornerstone of the file Manager
Take Option 2 a step further, and allow widgets to control the display of all file icons.
Widgets would register as handling certain mime-types. When a folder was opened with files of that type, the widget would open and handle the display of the icon.
For instance a widget for png's and jpgs would create a preview adjusted to the current icon size. It also might register rotate right and rotate left on the context menu (Right click on Widget) for the file manager to allow image rotation.
A widgets for movie files might have control buttons to allow the movie to be watched in place. Browsing away from the folder might cause the movie to pause. Upon returning to the folder, the movie would be paused where left off.
PDF widgets might have a next page button and previous page button, to browse through the document.
A default widget might diplay an appropriate icon for any files not handled by a more specific widget.