When viewing invoices you have the Preview & print and Email to customer buttons, well my idea is to perhaps have a check box that would enable/disable a drop down menu containing a list of watermarks such as PAID, OVERDUE etc.
When the check box is enabled then a watermark is selected, when you click on the Preview & print or Email to customer button, the selected watermark is added to the PDF.
Additionally the watermarks can be predefined in the Templates section of the settings menu. Different colours, sizes, orientations and text strings can be added, edited or deleted.
This is also something that could be further applied to the proposed quotation and payment receipt ideas.