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.
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.
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!