Currently, showing a dialog can take 10-20 seconds when started in DEBUG mode. 

And 3-5 seconds RELEASE mode. Improve the loading time.

A little way to speed up is to set ExpandToCurrentDirectory to False.

Like in MS Win 32 OpenFileDialog - thumbnails with preview or details about the selected file.

Check the attached images for better clarification.

Show the actual image when the layout of the main pane is set to Medium, Large and Extra Large Icons.

Add an API that can be used to cancel or modify the editing of files and folders. For example, you can introduce Editing and Edited events for the dialog which can be fired when the selected file goes into edit mode and after the editing is committed.

Currently, the root of the file system is "This PC". Allow setting a custom folder as a root (for example, C:\Users\mivanov\Documents) and show only its children.

Currently dialogs provide only custom places pane. 

Add auto-populated built-in section with well-known quick access like in the MS file dialogs.

FileDialogs are constructed by RadWindow and ExplorerControl as its content. 

Explorer control holds navigation tree, autocomplete, filtering combobox, main pane for viewing fodler and files, search box, breadcrumb. You can check all visual parts in the following help article:

http://docs.telerik.com/devtools/wpf/controls/radfiledialogs/visual-structure

Users might need to put the ExplorerControl in non-window parts of their application (for example in docked RadPane of RadDocking. Also, they might need to override OK/Cancel commands for that purpose.

Provide a way to create custom Confirm dialog when overriding a file in SaveFileDialog.

Setting the InitialDirectory property of the FileDialog to a folder with a huge large number of folders/items/files inside will significantly delay the loading process.

Provide built-in support for the following combinations:

• Move to the previous folder:  Backspace or Alt+Left Arrow combination
• Move to the next folder: Alt+Right Arrow combination
• Change layout: Ctrl + Mousewheel combination

If the OneDrive folder is available, display it in the navigation tree of the ExplorerControl.

Currently, this can be done manually by adding the OneDrive directory in the CustomPlaces collection of the dialog.

Click on Drive / Folder in the Navigation tree should put its Name in the AutoComplete box and then pressing the Open Button of the dialog should choose the selected drive / folder and close the dialog.

TextSearch navigation in the Main Pane does not work. If you press letter "a" the first folder with Name starting with "a" should be selected.
This needs to work in alll layouts including Details which uses RadGridView.

Performance degradation is observed while trying to open a folder in the file dialog with thousands of sub-folders/files.

Add an API that allows you to access the columns of the corresponding FileBrowserGridView control (representing the Details layout) so you can add different settings. Or alternatively provide some property abstraction to propagate properties from the file dialogs API to the internal RadGridView columns API.