As you did with PropertyInfoFieldInfo, which expose PropertyInfo property.

This can be very helpful to many custom logic based on the source PropertyInfo.
For example:

private static DisplayAttribute GetDisplayAttribute(QueryableFieldDescription fieldInfo)
{
    return fieldInfo.PropertyInfo.GetCustomAttribute<DisplayAttribute>();
}

I have Written behavior to auto-format the AggregateDescription.StringFormat.

Now, I need to list all possible AggregateDescription's, with reference to their DLL.
if (e.Description is LocalAggregateDescription)
{
    (e.Description as LocalAggregateDescription).StringFormat = MyStringFormat;
}
else if (e.Description is QueryableAggregateDescription)
{
    (e.Description as QueryableAggregateDescription).StringFormat = MyStringFormat;
}
else if (e.Description is Other_XYZ_AggregateDescription)
{
    (e.Description as Other_XYZ_AggregateDescription).StringFormat = MyStringFormat;
}

But, if IStringFormattableAggregate was public, I would can write it better:
if (e.Description is IStringFormattableAggregate)
{
    (e.Description as IStringFormattableAggregate).StringFormat = MyStringFormat;
}

See the attached image.

Steps to reproduce:
1. I checked the checkboxes 'Product' and 'Price'. The Product field added to row labels, and price to values.
2. Unchecked the 'Price', the price field removed from the pivot.
3. Unchecked the Product, doesn't  remove the products from the pivot.

 When there are a lot of items, it is quite difficult to find the ones you need in RadPivotFieldList. There could be some way to search within the properties.

Add options for showing items with no data as in Local data provider.

 Implement: "% of Parent", "% of Parent Row", "% of Parent Column"

Users should be able to navigate through the elements of RadPivotGrid using a keyboards. For example, moving between cells, collapsing headers with space, etc. 

In the RadPivotGrid:
Open the Label Filter dialog on a dimension, don't set anything and press OK, then all items are present and nothing is filtered out as expected.
Open the Label filter again and manually specify that the value must equal "xyz" and click OK then it will exclude every item that doesn't match this. Again, this behaviour is what you'd expect.
If you then open the dialog, and delete the "xyz" from the textbox so that the dialog appears exactly as it did originally, and click OK then you won't see any items in your pivot grid. This is because it is now filtering out anything that doesn't have an empty string as the label. This is not what I'd expect.
To remedy this, you can click on the dimension and see that the Label filter is still ticked in the context menu, and click "Clear filter", but this isn't exactly intuitive. 
I'd like the filter to be cleared when you clear the text in the box. 

Implement sorting by clicking on headers, like in RadGridView.
Unfortunately for Telerik, Infragistics have this feature:
http://help.infragistics.com/Help/Doc/WPF/2015.1/CLR4.0/html/xamPivotGrid_US_Sorting_Data.html

It happens when you change the distinct values selection several times. The PivotGrid disappears after the exception is thrown.
I managed to reproduce it in the demo by repeating this action around ten times.

Currently the header of that column says "Grand Total" instead of the aggregate name.