We can very easily add XML. We choose json instead as it is more widely used, especially sure cloud services. Json and mqtt were required and were added.
If XML is needed and someone can show why it is a must, then we can add it. By the way, there is already community work that ported XML and glide to TinyCLR. But if course if XML is needed then we want an officially supported library. Here is is
I will present your request to the team on Monday.