This functionality would aggregate a lot of value to the RadTreeList.  I will explain you why it is important. In order to prepare the information to give it to the TreeList, it is necessary to make a lot of hierarchical calculations that consume a lot of CPU and memory resources.  It causes that any server could be busy doing the same complex calculations with each click expanding any level. If you have a lot of users that expands a node of the tree, the server could crash. I prefer,  RadTreeList had all the information one time to prevent making unnecessary recalcularions with every click. 

I like Telerik components, but this issue make me to search another tree component in the market.  Please reconsider this functionality in your next version of your RadTreeList.

RadTreeList control dose not support RTL format, please add this feature.

check attachement.

Allow client side expand/collapse for RadTreeList