Orientation, margins, scale to fit one page, paper type, header/footer, etc.
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.
In addition to the fast document generation, the SpreadStreamProcessing can be adapted to import documents significantly faster than SpreadProcessing.
Add support for creating Tables and applying tables styles (predefined ones, similar to MS Excel, or custom).
Add API for appending data to existing worksheets. This way data could be filled in template documents.
Sheets can have Visible, Hidden and Very Hidden visibility. Add support for setting this property.
Expose an option to overwrite a sheet if it already exists in the workbook.
At the moment only the XLSX and CSV formats are supported. It would be a good idea to add the TXT format as well.