To be able to change the branding from Telerik to our own company or product specific banding. For instance MyApp Report Server with own images.

I would like to see a control as in an addition to the kendo mvc toolkit where you could build into an application the ability for customers to design the layouts of their reports.  For example.. Say you have an app you built for multiple customers.  Now the app might have a series of basic report templates in terms of queriable object model outputs. Being able to build an interface that would allow the customers to design their own layouts for what fields, positioning on the page etc..labels etc. and save that design to their account for future consumption would be a huge selling feature for an application. 
Specific example an invoicing application.  Well you could pre build a bunch of templates and have the customer choose one (the only real option currently available)...or you could provide a list of fields(based on object models for the underlying report query) and have an embedded designer allow them to design their own invoice and save it.  

At present one cannot have a subreport in report header/footer. This is such a limitation 
e.g. you have a company header which you want in every report. Normally what you expect is to make a subreport and include that in header of other reports. So if you change any details you can edit that subreport and it get reflected in all other reports. But unfortunately you have to include the company header in every report not only the controls but also in the datasource.
I used to develop in MS Access back in 90s and even then its reporting has this feature as standard.
So come on Telerik 

Having a white background in a view when using the dark theme is very bad for the eyes.
You should check all your UI in Visual Studio, as I already asked in the past for a correct context menu for dark theme (now done).

Currently, the size and location of items displayed in Properties view are rounded value, but the real value written in the .Designer.cs isn't rounded at all.

This created some imprecision in size and location, possibly resulting in unwanted resizing of the report width, or in overlap.
Now, it is only possible to correct this in .Designer.cs, and the user don't see the problem without zooming at maximum or opening the .Designer.cs

Rounded value seems better than some unneeded decimal, but then the value displayed should also be written in the .Designer.cs

When selecting several items, it should be possible to change the size and location of all of them, at least if the values are the same

1.  To add box plot chart feature which shows Statistical Five Number values: Median, Q1, Q3, Min and Max.
2.  To support Points Jitter which shows points of a data set (which are used to calculate Statistical Five Number) on same chart.
3.  To show label on X-Axis of the grouping.

Add the functionality of the Page Up/Down as well as Home and End to the WPF Viewer Control, so that the displayed page may be controlled using the keyboard.
These buttons should also work with the CTRL+ variations.

Datadynamics was purchased by GrapeCity some 4 years ago but Telerik Reporting is still relying on the old assemblies for converting.  The converter needs to be updated and should support rdx as well as codebehind files.  I was able to convert the Telerik.ReportConverter.ActiveReports to use ActiveReports 7 but am not able to use the modified dll.  See attached.

As reports are commonly based on dates, some means of doing date addition and subtraction would be beneficial.

Make the F5 key (refresh) in the Report Viewer programmatically controllable by using events tied to the viewer.  For example, I want to disable the F5 refresh functionality and add my own functionality when the F5 key is pressed.  I currently have the ability to disable the refresh icon (ShowRefreshButton=false), but not the F5 key button.

Would it be possible to prioritize which characters can "Break" when wrapping text inside a text box?  For example "foobar/foobar" will not break at the slash, it'll instead break at the character that hits the width limit. It would be my suggestion that punctuation be prioritized for "breaking" before all other characters, in the event that there is no space.

In the attached example, the column headers are dynamic (localized), and there's no way to predict what the text will look like (in whatever language)... Though I accept that no matter what, a string longer than the textbox width will need to break somewhere, I'd prefer that you break on "better" characters if they are present.

(Pretty nitpicky, I know, but this would save me trying to code the logic into an expression or function... which could prove complicated due to variable width of fonts)

When using the Report Explorer, the tree automatically expands to all levels expanded, on making any change to a Property for the currently selected item.
This makes using the Report Explorer very frustrating, so you end up shouting at the window to 'Stand Still!".

Support for Apple Numbers to be able to open reports exported in the "Excel Worksheet" format.

Our client is strongly requesting a "zoom" by graphical selection feature, as available in their former tool qlikview (or amcharts,  for example).
I am surprised this feature has not been requested yet, since any user would greatly benefit from it.
It's an extremely powerful and intuitive way of restricting your selection of report input parameters in a visual manner. 
By drawing a rectangular selection with mouse over the graph, you restrict the range of both x and y axis to the selected ranges, allowing quick zoom to the area of interest
Examples: just draw selection rectangles on a chart in one of the examples below
https://www.amcharts.com/demos/scatter-chart/

http://us-b.demo.qlik.com/QvAJAXZfc/opendoc.htm?document=qvdocs%2FSocial%20Media%20Buzz.qvw&host=demo11&anonymous=true

Add a StyleName property for the NoDataStyle so it can use styles from an external stylesheet.

I've seen some suggestions similar to this, but none that are exactly the same.  I would like to have the ability to select any item within the Report Explorer and drag/drop it to the location that I want it to appear.  Perhaps this isn't possible due the way the Report Explorer works, but this would allow greater flexibility in where a control is located, versus the current Bring To Front/Bring To Back options.