A TreeView/TreeMenu-like control would be great to have. Currently there is no easy way to realize tree-like structures, like the menu that the official Mail App has.
I think that will be a very good feature. Just create a gantt control available over this platform. A gantt control like UI for Silverlight will be a fantastic one.
Make RadDiagnostics Universal control.
I've ported over my app from Windows Phone 7/8, but now I've found myself at a loss of functionality without your textboxes available as universal controls. RadTextBox, RadPasswordBox, and RadNumericUpDown are all controls that would be fantastic if they could be ported over to Universal. Thanks!
The RichTextEdit from Microsoft is still not a perfect port, and missing many functionality. And only a few .NET vendor that has RichTextEdit control in their Windows Phone+WindowsRT suite, Telerik unfortunately not included, :( Please reclaim your crown as most complete Windows Store Suite back by adding RichTextEdit control for Windows Universal.