An open-source version of System.Xaml would provide the .NET community with an expressive, extensible, tool-friendly, and fairly mature framework for the creation of sophisticated object graphs.
While XML and JSON are both options for simple graphs, they lack the additional flexibility and power of features like markup extensions and attached properties, which greatly enhance the capabilities of the framework.
XAML has proven its broad applicability both inside Microsoft (Windows Workflow Foundation in addition to System.Windows/Windows.UI.Xaml) and outside it (Xamarin.Forms). I personally would have liked to be able to use it in .NET Core and Xamarin for a non-UI project. It seems likely that more projects would adopt XAML if it were more broadly available, and an open-source version would help keep all implementations compatible.
Additional features built on top of some XAML-based systems, such as data-binding, could also be very useful even in non-UI situations, but they wouldn't be required for System.Xaml to have great value.
Note: reference source for System.Xaml is already available at referencesource.microsoft.com, but this doesn't get us any closer to a portable or cross-platform implementation.
Real VB6 programming and VBA programming commented
A company like Microsoft should not take a language or development tool away from its users.
Oh man. OK. I had 6 comments on this thread, which seems a little silly now that I re-read them all. I have another post, so I will delete them all and replace with this one.
First off, official recognition (but NOT commitment) towards this can be found here: https://github.com/dotnet/corefx/issues/5766
You can also see a discussion around this very subject that has been going on for nearly a year here: http://forums.dotnetfoundation.org/t/nature-of-core-net-system-xaml/968/21
Also, here are current community efforts at a cross-platform port:
https://github.com/SuperJMN/OmniXAML (nice write-up of features here: https://grokys.github.io/perspex/perspex-alpha3/ )
Finally, you can track this vote along with others via weekly reports found here:
Incidentally, this console application was built/defined using Xaml, which can be seen here:
Long Live Xaml!