Currently term store operations are only available via JSOM. It would be nice to see this added to the REST API as well.

Support .NET Core with CSOM

Currently it's not possible to set the List.NavigateForFormsPages property through CSOM as it is in the server API.

You cannot currently use CSOM to publish content types in a content type hub in O365. Right now you must imitate the page post back to do it.

App-only policy in the SharePoint App Model is very restricted regarding Taxonomy operations. Even if an app is granted full control over the tenant, it lacks the permission to do anything but read operations on Taxonomy. Please provide the ability to create Groups, Term Sets and terms using app-only policy.

Please provide subscriptions API (_api/Web/Lists('listID')/subscriptions support (WebHooks)) SharePoint 2016 On-Premise which helps in webhooks, desktop notifications, mobile/push notifications and cross platform notifications to name a few.

It is actually possible to import the definition of a XsltListViewWebPart via LimitedWebPartManager in CSOM. However, not all properties are supported. Important properties like XmlDefinition, Toolbar, ViewStyle, .. are being ignored. There's no possibility to have everything set correctly not even manipulating the backed view after importing the WebPart. Please read also this discussion:
http://sharepoint.stackexchange.com/questions/90433/add-document-library-xsltlistviewwebpart-using-csom-or-web-services

Provide option to enable\disable event firing from Remote Event Receivers in SharePoint Online (CSOM/API) similar to SSOM - EventFiringEnabled

It's currently possible to identify the features that are activated at the Site and Web level using CSOM, it isn't however possible to determine which features are present but not activated using CSOM. It would be great if this feature could be added!

Need to do this in CSOM
$columnDefaults = New-Object Microsoft.Office.DocumentManagement.MetadataDefaults($list)
$columnDefaults.SetFieldDefault($Folder.Folder, "TestMMSColumn", "1033;#" + $term.Name + "|" + $term.Id)

Add support for managing information management policies, and especially retention policies, on content types and in libraries using CSOM. These are the objects largely in the Microsoft.Office.RecordsManagement.InformationPolicy assembly, such as the Policy, PolicyItem objects, etc.

Actually, the only way to paginate list items using the new REST APIs is to use __next property. There's no way to have a full working pagination using $top and $skip token as for any other REST call.
See my comment here:
http://msdn.microsoft.com/library/d4b5c277-ed50-420c-8a9b-860342284b72.aspx

At the moment it does not seem possible to create WebPart connections via the Client Side Object Model.

There's currently no better way to configure search indexed and managed properties than export/import configuration. It's a huge pain when building search related apps and a black box, we need something better

When creating Site Collections through CSOM and providing an LCID. (For example 2067) an exception is thrown.

[Quote]
SiteCreationProperties.Template\r\nParameter name: Specified value is not supported for the SiteCreationProperties.Template parameter." is thrown at ExecuteQuery();
[/Quote]

The Exception is also not pointing in the right direction.
1) It should be possible to create Site Collections providing the LCID
2) When an Invalid Lcid is passed, the exception should point in the right direction.

Some list/library settings are available via CSOM. Others are available via the list property bag (although that approach is not desirable). However, there are even other advanced settings that don't appear to be accessible (read or write) via CSOM/REST. In particular, the advanced settings for "open documents in the browser" (open in the client application, etc.) or allow "quick edit" (yes/now).

I'm looking at SharePoint 2013 when trying to do this, but I suspect the same problem occurs for SharePoint Online and SharePoint 2016.