Add support for the All Caps and Small Caps properties of the runs. These properties are currently ommitted on import.

If a worksheet spans more than one page, you can print row and column headings or labels (also called print titles) on every page to ensure that the data is properly labeled.

When exporting a document with the .NET Framework implementation, only subsets of the used fonts are embedded in the document. 

This is currently not available in the Silverlight implementation, and it most probably won't be available in the .NET Standard implementation, as the current implementation is dependent on the WPF font classes.

Provide API to plug similar logic when using different platforms as well.

Most often, the print area will be defined in the Workbook part like so:
<definedName name="_xlnm.Print_Area" localSheetId="3">Data!$A$1:$L$7</definedName>

Sometimes, however, it can be a function instead:
<definedName name="_xlnm.Print_Area" localSheetId="0">OFFSET('Parametrový List'!$A$1,0,0,INDEX({77,148},PocetStran),34)</definedName>
At the moment, the second option is not supported.

The OFFSET function returns a reference to a range that is a specified number of rows and columns from a cell or range of cells. 

Enable the customers to import SVG images and use them in their documents.

For instance, we may implement WritePageAsync method in addition to the existing WritePage method. This is a similar scenario as in System.IO.StreamWriter class which has WriteLine and WriteLineAsync methods.

Add ability to create, import and modify layers (optional content) in a PDF document.

When a format code like "General" x"" is applied on e.g. 1, the result is "1 x". At the moment, the result with Spreadprocessing is "General x"

Provide the customers with the ability to export workbooks to HTML files.

Implement support for footers in HtmlFormatProvider.

Introduce new section properties which are responsible for defining columns in the section.

TC field has special tag - \tc, which is currently not supported and is skipped during import. Because of this, the argument of the field, which is hidden in MS Word, becomes part of the document content.

Enable the customers to add digital signatures to the documents and read signed documents.

Currently, the align attribute is supported only for a table and when applied to a cell or row is omitted on import.

You can convert the document to PDF and use the approach bellow:

Currently, silent async printing may be achieved by using RadPdfViewer WPF control. Sample demo showing how to achieve this may be seen at the following forum post:
http://www.telerik.com/forums/pdfviewer-print-makes-ui-unresponsive#js0YdzFWc0Oa8C3g6a18lg

The RadPdfViewer WPF control used does not need to be displayed, making this demo a good workaround for ASP.NET AJAX clients.

In addition to the fast document generation, the SpreadStreamProcessing can be adapted to import documents significantly faster than SpreadProcessing.

Such objects are defined as oleObject elements in the XML. Currently, they are skipped on import.

The Indent property of Table is not respected by PdfFormatProvider.