In addition to the fast document generation, the SpreadStreamProcessing can be adapted to import documents significantly faster than SpreadProcessing.
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.
Add support for adding images to the exported document.
Add support for creating Tables and applying tables styles (predefined ones, similar to MS Excel, or custom).
Expose an option to overwrite a sheet if it already exists in the workbook.
Add API for appending data to existing worksheets. This way data could be filled in template documents.
Add ability to export charts.