For the current wave of new Win10 UIs it would be handy to have a new Control like the Windows 10 Splitview available in WPF.

See: https://msdn.microsoft.com/library/windows/apps/windows.ui.xaml.controls.splitview.aspx

Please provide something like the new RelativePanel-Control from Windows 10 UWP for WPF.

See: https://msdn.microsoft.com/en-us/library/windows/apps/windows.ui.xaml.controls.relativepanel.aspx

One requires to create QueryableCollectionView on the UI thread.
Now there's going to be a .Count() call on the IQueryable passed into the constructor,
which will block the UI thread until it returns.
There might be various reasons why there is delay (unable to connect to DB,slow query, using a REST queryable provider, etc).

It would be nice if we could pass in the count ourselves.

There is an open source library (http://bitmiracle.com/libtiff) that can be used to convert the TIFF images to different formats and then visualize the resultant file with an appropriate control (e.g. convert the TIFF to PDF document and import it in RadPdfViewer).

When developing live (localization/resource) display language switching for SL5, the LocalizationManager should allow to update all localization strings applied through the ResourceManager. In SL5 it's not possible to listen to an attached dependency property for all dependency objects. Therefore it would help to have a way to know when the attached ResourceKey dependency property changes (and on which DependencyObject) in the LocalizationManager. 

Create NuGet symbol packages - containing the .pdb files - corresponding to the primary NuGet packages. The process is described here: https://docs.microsoft.com/en-us/nuget/create-packages/symbol-packages .

This will facilitate the debugging of problems in the controls code.

Introduce support for AutoCad files in a fixed document viewer.

Introduce control similar to TextBox which offers integrated spell-checking (similar to the one provided in RadRichTextBox).

Add: ListView Control

Something like an image Viewer for WPF with some kind of animation, not too fancy though.

It would be very helpful if you can plan to create kind of a "drawing tool" to design specific views for flows, state models, workflows, etc.

Develop  a new control - Reflection Control.