Expose a property to easily control the expander indicator style and visibility. From Q1 2016 SP users will able to use the ExpanderStyle property of the RadpanelBarItem in order to style the arrow expander.
Allow resizing of PanelBarItems. http://www.telerik.com/community/forums/wpf/panelbar/resize-capabilities-to-radpanelbaritems.aspx
Add a ScrollViewer to the PanelBarItem ControlTemplate