The control should allow customizations in its filtering functionality, for example, the developers should be able let RadComboBox match "Á" when the user types "A".

Allow the developers to style the TextBox area of a editable RadComboBox - add icons, change the string, format the text etc.

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

Available in the 2016 R3 Release.

Please check our help article for more information: 
http://docs.telerik.com/devtools/wpf/controls/radcombobox/features/multiple-selection

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"

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;
		}                
	}	
}