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.