Currently, the RadRibbonTab GetChildrenCore method doesn't create automation peers for the element in the ContentPresenter for the TabStripAdditionalContent.

At this point, you can enable this by creating a custom AutomationPeer that derives from RadGridViewAutomationPeer and override its GetChildrenCore() method. This way you can manually create the peer for the AdditionalTabStripContent. This idea is shown in the attached project.

The sheet selector should hide sheets when their visibility is set to hidden or very hidden. The context menu of the sheet selector should contain Hide and Unhide options that allow to hide a sheet or unhide one of the hidden sheets.

HeaderSizeNeeded is fired twice with the same index and affects the header row width. 

Workaround:

int prevIndex = -1;
private void grid_HeaderSizeNeeded(object sender, Telerik.Windows.Controls.VirtualGrid.HeaderSizeEventArgs e)
{ 
    if (e.Index == 0)
    {
        if (prevIndex != -1)
        {
            e.Size = 10;
        }
        else
        {
            e.Size = 100;
        }
    }  

    prevIndex = e.Index;
}

Add an option to modify the StartPoint and EndPoint properties of RadDiagramConnection instances via the SettingsPane.

This functionality can be achieved by modifying the default ControlTemplate of the SettingsPaneView element to include an additional RadTabItem instance in the RadTabControl element. In the additional RadTabItem, input controls can be used, such as the RadNumericUpDown element. These elements should modify properties that will be present in a custom LinkViewModelBase<NodeViewModelBase>> class. More specifically, the X and Y properties of the StartPoint and EndPoint properties of the RadDiagramConnection element.

The attached sample project shows the implementation of the above approach for achieving this requirement.

Add an option to customize the available colors in the SettingsPane control:

The new button should be placed in the column container under the tasks/cards and it should add a new task.

This feature is the same as the "Add a card" button in RadTaskBoard for WinForms.
https://docs.telerik.com/devtools/winforms/controls/task-board/task-board-adding-task-cards

Currently, RadPdfViewer uses WPF DrawingContext class for drawing page elements to a Canvas.  When PDF page has many content elements (usually geometries) drawing the page may be time-consuming and freezes the UI. 

Instead, we should consider implementing another approach for rendering the content asynchronously which would improve the user experience.