Please allow for the creation of custom UI widgets and plugins that can be tied closely with the rest of the framework similar to how jQuery plugins work but better ;)

Start off by providing good documentation of the different classes (both JS and CSS) that developers can and should use and provide guidelines for writing custom code so third party controls and extensions follow similar style (visually and programatically).

Also adding a Kendo Marketplace or something like this where developers can publish their plugins and other can vote/comment on them would be great for everyone. 

jQuery UI and the plugins that are available there are great but I think Kendo UI and the Kendo community can do better especially if we set the bar a little higher from the get go.

Textbox control which keeps extending until we finish typing.
something like what iOS have when we want to add note in contact.

The new funnel in the Q3 release is great, but it looks very flat.
It would be great to have a more 3D look.

Once created, many Kendo components root DOM element are wrapped under new HTML elements.  Any class of the original input will also be copied to some parent elements.

As each widget (DateTimePicker, DropDownList, ComboBox...) are wrapped differently, it might be a good idea to add a class to the root input (k-root) that would allow us to search for that specific input.

See this post for more details:
http://www.kendoui.com/forums/ui/general-discussions/best-way-to-find-the-root-element-by-class.aspx

Create RichTextEditor Widget which is integral part of all content management systems. Also, it would by nice to create checkbox widget.

Really important to have support for ARIA roles on these types of widgets. That's what they were made for, and jQueryUI does a pretty good job at this.

Accessibility is important.

/*** Kendo UI Beta v2014.3.1029 ***/
We've tested the brand new TreeList today.
The new TreeList needs...
> checkboxes
> columns should be resizeable
> column reordering
> frozen columns
events
> collapsed
> expanded
Take a look at the "famous" Grid / TreeList...

Regards
Stefan Wolf

Using the Sortable Widget on a Grid works great to allow the user to manually sort the grid rows.  But if the grid has a vertical scrollbar, the  user has to drop the row multiple times to get a row from the top of the grid to the bottom.  It would be nice if the grid would Autoscroll when the user gets to bottom of the viewable grid.

Thanks,
Mike

React uses a declarative paradigm that makes it easier to reason about a JavaScript application. React computes the minimal set of changes necessary to keep your DOM up-to-date. 

Kendo UI seems not to have such an update feature. But since React works with most libraries and frameworks, please create an example app with React or make a blog post about how to use React together with Kendo UI Web.

Create light-weight Diagram visualization control/wrapper.It may integrate with Telerik's Silverlight/WPF RadDiagram framework.
http://www.telerik.com/products/silverlight/controls/diagrams.aspx

I really like what you've done so far and I think you're very close to the top if not already there in terms of being an HTML5 UI leader. The one thing your data visualization set is missing is a map. A simple map to show chloropleth data, basic paths etc is well within your reach and would round out your DataViz offering.

I would really like to see a Gauge control similar to what is available through the Telerik Silverlight controls.

It would be nice to use the Kendo UI Editor to manipulate and style tables, add/remove rows and columns, just like MS Word does.
Together with some friends we discuss this issue and we saw that it is a very important and useful feature for the most end-users, because they are comfortable working with MS Word and they notice that fault when using web applications.

It would be great to have file manager support in addition to the image manager for the HTML editor. Some sort of way to upload resources, organize them, crop (for images), and ultimately select from a picker to be included in the HTML markup. 

Would be nice to have a way to easily configure columns to be fixed (position and width), like a checkbox column to select rows.

This kind of column need to have fixed width and position (cannot be reordable).

But, KendoUI could has also a native support for rows selection, allowing us to just turn on this feature that could have a lot of sub-features: Remember selected rows by page, mark/unmark all, custom options in the column menu to massive mark, and so on.

Element to chose color. That is usefull forvisual  template bulders and CSS editors and more. I am curently needing this UI

Just test the basic usage sample on the web site.
If you are able to set all splitters as you wish, you are the king!
All other controls work as expected.

My marketing department has requested a specific look for a chart. The Moc up image is http://i.imgur.com/Z2hKaei.jpg in order to achive what they are asking I need the Labels to work in the Bullet Charts. 

It would also be great if you could add a label to the Target as pictured in the image. 

A ListBox widget with the following core features: skinnable, drag and drop to transfer items, ajax binding, support for templates.