Thank you for your proposal. [;)]

Creating a plug-in able to paste the color and texture of the clipboard items to the currently selected items could be a nice idea to start.
From the forum and the feature requests, here are other ideas requested by users that could be achieved with a plug-in:
- Save/Restore viewpoints (I foresaw to develop this in the coming months but you can create your own system if you want to).
- Join two selected walls.
- Split walls around rooms to ensure no piece of a continuous wall is shared by multiple rooms, and paint the walls around a room or compute the walls area of a room.
- Design the shape of a new room by triangulation.
- Import from various 3D formats like Blender, DWG (supporting additional format can also be done with a dedicated Java 3D loader).
- Export to other 3D formats like Collada, VRML...

To finish, I don't believe you can program a plug-in that will span to the grid items while the user draws them. In the current architecture, a plug-in can't modify how mouse behaves in the program.