Develop  a new control - Reflection Control.

Features to support:
- Text blocks with basic formatting, preferably draggable, and preferably with customizable borders/backgrounds.
- Basic drawing functionality such as pencil, brush, lines, circles, rectangles etc. Of course with color selection for lines and fill. 
- Line thickness selection. 
- Being able to move the objects around the board would be nice.
- Connecting objects with some sort of connector lines, pretty much like the map structure in SketchFlow, if you are familiar with that. 
- Some basic shapes, such as arrows and icons that can be dragged/dropped on to the board.
- Support for uploading images and dragging them onto the stage. 
- XAML export (necessary for saving a drawing)
- Printing
- Magic marker/highlight functionality
- Collaboration support. This is a big one, and I am not sure how it should be implemented. Full collaboration functionality out of the box would require a service running duplex WCF or something similar. Alternatively, perhaps Telerik could provide us customers with an API and some sample code. Then we could handle the back-end stuff ourselves.

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Our XAML team has recently reviewed the status of this feature request. Since most of this requirements are covered by our RadDiagram Control / Framework we are changing the status to Completed.
Please follow RadDiagram documentation:
http://docs.telerik.com/devtools/wpf/controls/raddiagram/overview.html

An advanced search textbox control for WPF. A good example can be found at: http://davidowens.wordpress.com/2009/02/18/wpf-search-text-box/

Create a contour plot control for Silverlight. http://mathworld.wolfram.com/ContourPlot.html

Develop Apple/MAC theme

Create a Button editor control

Add new control to the suite: Shuttle control.

Shuttle controls is a couple of ListBox controls and several buttons in between, that move items from one ListBox to another and vice versa. 

Here is an example of the interface:
http://www.mathworks.com/access/helpdesk/help/toolbox/comm/ug/shuttle1.png

Check the following two examples showing how Shuttle control can be easily created using RadListBox or RadTreeView control:
https://github.com/telerik/xaml-sdk/tree/master/ListBox/ShuttleControl
https://github.com/telerik/xaml-sdk/tree/master/TreeView/ShuttleControl

Create an Office2010 theme.

New TimeLine control featuring ZoomBar.

A component to easily chat with visitors on a website.  It's becoming popular to be able to interact and offer support that way.  Maybe based on the Windows Live Components or something along those lines.

I'd like to put in my request to have a control that would allow to magnify a part of the image as mouse goes over. Here is link to see a running sample:
http://www.fordracingparts.com/parts/part_details.asp?PartKeyField=11439
As you move your mouse over, another window opens up to show a magnified version.

Add new control to the suite: Wizard control.

A data form control that allows the users to rearrange the fields using drag-drop and then save the layout. This control would be used in information systems where the admin users should be able to create custom forms for the regular users.

LayoutControl is now officially available since R3 2016 Release.

Develop  a new control - Fisheye Panel.

New Control: WPF Web Browser

A useful new control would be : RadWebcam

It would show webcam view in realtime and call a function each x seconds (or ms), would be useful for :
 - user picture to attach to its user account
 - quick product picture
 - and for my need : call an external function for barcode recognization

I think it would be easy and fast to create this control for WPF, and it would fit perfectly with Silverlight 4 (or 5 ?) webcam support.