Please, consider next enhancements to Field Customizer (and Modern List UI):
1. implement modern Quick Edit mode
2. Add "edit" mode to field customizer
3. Make field customizers (both view and edit modes) applicable on forms, panels, quick edit mode, etc.

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.

Today we get PageHead & PageFooter... would like to have BEFORE/AFTER DIV's for `QuickLaunch` & `PlaceholderMain`

Cross-reference:
https://github.com/SharePoint/sp-dev-docs/issues/625

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.

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.

The latest adjustments in the Microsoft Store police allows store's add-in to works with conjunction with another add-in that is available only from the provider. The solution package (.sppkg) build with SharePoint framework is not classified as 'add-in'. Thus, we cannot add add-in in the Store that depend on SPFx solution and there is no way of delivering customizations to the modern page.
I can offered two options for solving this issue: 1) Adjustment of point 4.18 (https://docs.microsoft.com/en-us/office/dev/store/validation-policies#bk_4) from the Store policy to allows SPFx modifications 2) ApPSource store to start allowing .sppkg solution packages to be added

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

Please provide a well-known placeholder for the property pane or better a similar placeholder that let us create a sidebar.

Having this ability would let us create powerful sidebars/toolbox extensions. This would be a super cool enhancement!

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.

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

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

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!

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.

List items that have an Audience field attached should be audience filtered when retrieved via the REST API.

For example, a list that has link items with the Audience field attached to target particular items at particular groups of people. If the REST APIs are used to retrieve items from this list, then the identity of the requesting user should be optionally used to filter the results based on audience.