Currently RadComboBox supports filtering of the data when IsEditable is True. 

Add a mode where a search box is displayed (when IsEditable is False) in the drop down of the control.  The searchbox should filter the items in the dropdown.

Currently filtering with virtualized RadComboBox is not supported as the filtering feature works with the containers (changes their Visibility).

Currently, the drop down of RadComboBox contains only the RadComboBoxItems and the Clear Selection button. Add new content presenter that allows you to include extra visuals, like buttons or text.

The feature is similar to the AdditionalContent of RadTabControl. 

Setting the Background property of RadComboBox no longer changes the background color of the control.

To work this around, you can add a global event handler for the Loaded event of RadComboBox and set the corresponding backgrounds in code.

static MainWindow()
{
	EventManager.RegisterClassHandler(typeof(RadComboBox), RadComboBox.LoadedEvent, new RoutedEventHandler(OnComboBoxLoaded));
}

private static void OnComboBoxLoaded(object sender, RoutedEventArgs e)
{
	var comboBox = (RadComboBox)sender;
	if (!comboBox.IsEditable)
	{
		if (comboBox.IsEnabled)
		{
			var buttonChrome = comboBox.FindChildByType<ButtonChrome>();
			buttonChrome.Background = Brushes.Purple;
		}
		else
		{
			comboBox.Background = Brushes.Purple;
		}                
	}	
}

The RadListBox has an attached property for mvvm binding of SelectedItems - https://docs.telerik.com/devtools/wpf/controls/radlistbox/features/selecteditemssource.

Please support SelectedItemsSource for RadComboxBox too.

If you set IsEnabled = false, in the RadComboBox then (in the Office 2016 theme) the entire box is faded, including the text. A suggestion in the forum was to set IsHitTestVisible to false, as well as IsTabStop. This works, but it means that the dropdown button still looks enabled. It would be nice if IsReadOnly did what I wanted, but it doesn't. 
Therefore, either a way to set the Visibility/Opacity of the dropdown button. Or alternatively, modify the theme(s) to make the text more visible when the box is IsEnabled is set to false, in the same way as the standard ComboBox. 
I'm aware that I can modify the template myself, but it'd be nicer if it was built in. 

Combobox with 2 items -- "Item1"   and "it": typing "it" selects the 1st item which is "Item1" and it should select the second one which is "it"

It might also be nice to take it one step further and give the option to choose the key that fires an update off

Currently the control scrolls by group.