The customers should have the ability to change the symbol used as a delimiter and the symbol used as a quote for a CSV document.

Orientation, margins, scale to fit one page, paper type, header/footer, etc.

Provide a way to export filters.

Such settings are: "Summary Rows Below Detail", "Summary Columns to Right of Detail" and "Automatic styles".

Add support for adding images to the exported document.

Add support for CSV settings when exporting (delimiter, encoding). 

Add support for exporting content with strikethrough font effect.

Add support for creating Tables and applying tables styles (predefined ones, similar to MS Excel, or custom).

Currently, this functionality is supported by the SpreadProcessing library. More information can be found in Worksheet Protection and Workbook Protection help articles.

Add ability to export charts.

Add API for appending data to existing worksheets. This way data could be filled in template documents.

Expose an option to overwrite a sheet if it already exists in the workbook.

In some scenarios, the customers need to apply (number) formatting to the whole row or column at once. At this point, formatting can be applied only to the cells.

Add support for Comments.

Sheets can have Visible, Hidden and Very Hidden visibility. Add support for setting this property.

Allow the customers to work with XLS documents.

Add support for importing files with absolute relationship target paths.

At the moment only the XLSX and CSV formats are supported. It would be a good idea to add the TXT format as well.