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).

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.

SpreadProcessing has a method,  cellValue.GetValueAsString(format), which allows the user to apply excel format on a value and get the result. While in SpreadStreamProcessing one could get the format and the value, there is no way to get their result and this is very inconvenient when the values in question are dates.

Workaround:

foreach (ICellImporter cell in rowImporter.Cells)
{
    string value = cell.Value;
    var format = cell.Format.NumberFormat;

    var cellValueFormat = new CellValueFormat(format);
   
    ICellValue cellValue;
    CellValueFormat valueFormat;

    CellValueFactory.Create(
        value,
        worksheet,
        new CellIndex(0, 0),
        cellValueFormat,
        out cellValue,
        out valueFormat);

    var formattedValue = cellValue.GetResultValueAsString(cellValueFormat);
}

Add support for Comments.

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