It depends on what type of indentation you want to achieve. For full flexibility FlowDocument with Paragraphs is required because it exposes TextIndent (first line indentation) and Margin properties. FlowDocument is a feature we want to implement (#1765) at some point but we are focused first on FlowDirection and RTL languages.

Anyway, for simple list indentation you can get something good using an ItemsControl, for example like this xamltoy:



Is this what you are looking for?

RichTextBox is more like implementing a fully featured editor and is not in our plans yet.