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

Provide a way to export filters.

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.

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.

Allow the customers to work with XLS 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.

Files exported with SpreadStreamProcessing cannot be read by OleDB consumers

Add support for Comments.

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

Allow users to define sorting.