Thanks @wyshmily for filing this request! We could potentially start discussing the implementation of this once we've finished open sourcing the rest of the wpf codebase. Do you have an idea of what you'd like the API to look like?

Just like Control.FontSizeProperty, TextBlock.FontSizeProperty, TextElement.FontSizeProperty,
I think there should be Control.LetterSpacingProperty, TextBlock.LetterSpacingProperty and TextElement.LetterSpacingProperty

<TextBlock LetterSpacing="5" Text="Some Text" />
<TextBox LetterSpacing="5" Text="Some Text" />
<Grid TextElement.LetterSpacing="5"></Grid>

I think this was implemented in Silverlight, but I just can't find the API reference, it was somewhere here before: https://docs.microsoft.com/en-us/previous-versions/, maybe the API and some implementation could be reused.

I think it was this one: Control.CharacterSpacing Property

It still very demanding feature. Below sample from Figma.

I'm also interested in how to achieve custom character spacing. All answers I found online either didn't take into account original kerning, or were far too complex or inefficient. I believe the best way would be to add (or subtract) to existing kerning, since kerning can be disabled using Typography.Kerning="False" if needed.