Sometimes when more than one workbook is opened, then a:
GeneralException: There was an internal error while processing the request
is thrown during various office-js operations.

See https://github.com/OfficeDev/office-js/issues/771 for details.

I know there's a workaround:
https://docs.microsoft.com/en-us/office/troubleshoot/excel/force-excel-to-open-new-instance
but it's impossible to force clients to use it.

At the moment addins run in EdgeHTML

Recently Chromium-based Edge has been released which is much faster than EdgeHTML

Add-ins should run in new, Chromium-based Edge.

Will it be "just done" at the next Windows update?

Similar to previous suggestion:
https://officespdev.uservoice.com/forums/224641-feature-requests-and-feedback/suggestions/31155925-office-add-ins-should-run-in-edge-not-ie11

Microsoft have decided to make the E5 Developer Subscription available to only new developer subscriptions. Can this be made available to existing developers?

https://developer.microsoft.com/en-us/sharepoint/blogs/new-microsoft-365-e5-subscription-with-ems-now-available-for-developers/

Function/ability to trace precedents and dependents with the office.js API (similar to GetPrecedents in VBA).

It would be useful to allow Outlook Add-ins to have context menu extension points to allow for more granular action controls (e.g. Right-clicking THIS specific email allows me to act on it with THIS add-in)

It seems that the context menu isn't off limit to all Office programs, such as Word or Excel, but there is not any support for Outlook.

https://dev.office.com/reference/add-ins/manifest/extensionpoint

We want to set a different signature in the mail body from the outlook add-in. Right now altering the whole html of the mail body makes this quite impossible, especially because it is different on all different platforms. (win, mac, online different browsers)

It would be extremely helpful in certain cases to prevent user to interact with the sheet/workbook while some action in the task pane is required or when some process is running (e.g. when some complex sheet changes are being performed - formatting, inserting columns, inserting data, charts....).

Need a Change Tracking API in Word.

VBA COM Object model supports Application.CompareDocuments to compare and merge word documents. Office.js lacks this capability. Please support this through Office.js

It would be very useful if add-ins dropdown menu could contain dynamically generated items. Now the displayed content in the ribbon dropdown need to be hardcoded even for sub-menus which limit the capabilities of add-ins commands.
See question on stackoverflow
http://stackoverflow.com/questions/36315571/dynamic-content-in-add-in-command-with-ribbon-dropdown-menu

In a workflow scenario (not sharepoint workflow, something custom) where we want to have a button in a Word Add-In taskpane to close the Word Online window after saving the file. In this button we initiate a next workflow step by saving the document, initiate the next step by e.g. setting a flag in sharepoint and closing the document editing session of the user.

This could either be an office general api or a word specific api (this is where we need it the most).

Currently an add-in command will disable if multiple Outlook mail items are selected. It would be really useful to be able run certain commands by iterating over a collection of the selected items.

Our very successful commercial Outlook add-in ePredict for SharePoint (http://ww3w.docauto.com/product/epredict-for-sharepoint) automatically launches its UI in the task pane in desktop versions of Outlook. Our customers are interested in a version for Office 365 Outlook online. However, Office 365 Outlook online requires user input to launch a task pane add-in. Our users would not accept this. Please add functionality, probably in the manifest, that would allow a user to give permission to an add-in to automatically launch in the task pane when an existing email is opened or a new email is being created. Thanks!

We are able to invoke a built-in Ribbon command in VBA by using:

Application.CommandBars.ExecuteMso(“Copy”)

Could we have a way to invoke built-in functionality from Javascript

Recently we got an option to insert task panes using add-in commands which is really nice feature. It would be very useful to have an option to insert content panes in Excel, Word and Power Point as well.

The current BindingDataChangedEventArgs object passed to the event handler for the Binding.BindingDataChanged event only contains properties for the binding that raised the event and the type of event. There is no specific information about what cells, columns, or rows in the table object were changed. When dealing with tables with many rows and columns and/or end-user gestures that edit/insert/delete many cells/rows/columns at once, this API is not very useful. Contrast this to the (Excel) COM APIs where, at least, the SheetChanged event provides a Range object that describes what was changed.

Can you please provide a feature which enables to create the bindings to named range which has multiple non contiguous ranges in it ?.

The new Mail Apps make adding functionality to OWA very easy, however they lack some of the power of a full-on Outlook addin. I suspect that a common use case will be to create a full-powered addin, and a "lite" mail app that handles what it can for OWA. At present, this doesn't work because the mail app shows up in both. Furthermore, it is possible to create functionality that only applies to one client or the other. It should be possible to specify an app as belonging to OWA or to Outlook.

See also http://social.msdn.microsoft.com/Forums/office/en-US/13c3413d-7f3f-4e7c-9b6d-c20e95b064e3/how-to-enable-mailapp-only-for-owaoutlookwebapp-not-for-outlook-client?forum=appsforoffice