A control that is able to display PDF files natively in Universal App
Port the Windows Phone ImageEditor to WinRT. http://www.telerik.com/products/windows-phone/overview/all-controls/image-editor.aspx
I think that will be a very good feature. Just create a chart control available over this platform. A chart control like UI for Silverlight will be a fantastic one.
A DataStorage that can be used for both in Windows 8.1 and Windows Phone 8.1 apps.