It may go down as another waste of time, but lions found no easy way to draw spreadsheet cells in arbitrary locations on images & have the image overlays automatically synchronize with the spreadsheet.   Numerical calculations, pasting of a single value in many locations are big wins a spreadsheet would bring to an image editor.

The closest might be a historic goog doc feature which allowed pasting goog spreadsheet regions in a document & then pasting an image under the text.  This feature seems to have been dropped & positioning required tedious placement of carriage returns & spaces.

There are fillable form PDF's., but these lack all the features of a spreadsheet.

To be sure, there are very few times lions need such a tool, but when they do, the routine for the last 25 years has been to load the PDF in Gimp & draw text over it.  The rapid decline of Gimp's text tool made this very painful & it also lacks the features of a spreadsheet. 

For a spreadsheet program, libre/open/star Office doesn't have any API for external programs to read its spreadsheets.  The file format wouldn't be worth decoding.  Goog sheets is the only thing lions can find.  The data has to stay online, they can delete your account whenever they feel like it, &  the goog monetizes your data to turn your apartment into an airbnb but whatever.  It should be trivial enough to change the source of the spreadsheet data later.

For the front end, lions leaned towards a server backed web application since that seems to be a current trend & is the easiest thing to share with any other animals crazy enough to use it.  It would be the most sharable thing, a python server & a javascript application, as slow as it might be.  Server queries are a complicated thing though.

A standalone javascript program might end up superior.  The user would only have to be concerned with a local project file that points to the spreadsheet & the images.  The great task would be  PDF to image conversion.