Add support for shapes, and especially for shapes with textual content. 
In OOXML, shapes are represented by the wps:wsp element, and shapes with textual content by <wps:txbx>, <w:txbxContent>.
Such shape can be added to a Word document using the Insert -> Text -> Text Box -> Draw Text Box, or through a shape's context menu -> Add Text.

Do not confuse these shapes with Text/Rich Text content controls (see https://feedback.telerik.com/Project/184/Feedback/Details/190057 )

Allow to password-protect a workbook, so that it cannot be shown (read) without the password.

Currently, signature flags, represented by the 'SigFlags' optional entry, are not imported and exported, and there is no API for setting them.
As a result, documents signed with RadPdfProcessing (or with RadPdfViewer) appear in Adobe Reader without the blue document message bar at the top (the analogue of the RadPdfViewer's SignaturePanel).

If the API is supported, setting the flag to 1 or 3 would ensure that the message bar would be visualized in Adobe Reader.

Currently, indexed colors are imported only regarding the default color palette. Instead, when the entire palette is predefined using a construct like: 

  <colors>
    <indexedColors>
      <rgbColor rgb="00000000"/>
      ...

in styles.xml part, the indexed should be changed with the provided ones. As this feature is not implemented, such colors are changed during the import.

With the current implementation when exporting a Stitching function all the containing functions are exported as Sampled functions even if they originally have been imported as Exponential interpolation functions, which leads to an increase in the size of the document.

More information can be found in the PDF Specification.

This attribute specifies the alternative text for the current DrawingML object, for use by assistive
technologies or applications which do not display the current object. Currently it is omitted while importing the content with DocxFormatProvider

The WordsProcessing library doesn't support the import of image alternate attribute ("alt").

DataTableFormatProvider: One should be able to set the AllowDBNull property of each column.

When this type of section break is used, the next section should start on the same page instead on the next one.

Add Mail Merge events.

Introduce support for the WEEKDAY function.

A feature in Excel allows adding ActiveX controls in the Worksheets. You can insert the ActiveX controls from the Developer ribbon tab from the insert button. As it is a missing feature it is not imported and thus removed from the document on export.

Currently RadFixedDocumentInfo is used only for export of Author, Title and Description document metadata properties. This should be extended to support custom properties. We should also implement the import of RadFixedDocumentInfo.

Implement support for cell references to columns and rows, e.g. $1:$2, A:D, etc.

Currently, CellSelection.SetValue throws ParseException, and CellSelection.SetValueAsFormula throws InvalidOperationException. 

Handle the case where the document has edit, copy password but no open password. We should either suppress the UserPasswordNeeded event or provide information about the password options in the arguments. 

Implement functionality to support scenario when a document which already has a signature to be signed again or add two or more signatures.

According to the PDF Specification: The definition of an indirect object in a PDF file consists of its object number and generation number, followed by the value of the object bracketed between the keywords obj and endobj. For example, the definition:
An invalid case:

According to PDF specification, all fonts should be embedded except from 14 Standard PDF fonts. This guarantees that the PDF file is self-contained and will be rendered successfully on different devices and operation systems.

However, some clients prefer not to embed fonts in order to reduce file size. That is why we may provide such API.

In newer versions of EXCEL (2019 and later) the CONCAT function is introduced as a replacement of the old CONCATENATE function. It adds the ability to specify a range of cells when generating the resulting string in addition to its previously supported input arguments.
NOTE: In EXCEL starting from version 2019, CONCATENATE can no longer be inserted in the worksheet through the UI, it can only be used if you type it directly in the cell's contents.

When importing a document with an unclosed <br> element in the vmlDrawing part an exception is thrown: System.Xml.XmlException: 'The 'br' start tag on line 183 position 14 does not match the end tag of 'font'. Line 184, position 7.'