Custom WPF editor using the Visual Studio Shell

Question

For the project I'm working on, I want to have a custom form designer.

I want to use WPF to create custom XAML. I don't know exactly which changes will be in the XAML but it will be custom XML tags. At the moment if I add those tags in a XAML file and try to edit it with VS, I get errors.

The custom tag data would come from custom ToolWindows we want to implement.

At the moment I'm looking at the isolated Shell solution and I'm not sure if I need to use DSL or not.

Anybody has a hint at how I could use the WPF Form Designer and be able to save in a modified XAML format without having to re-implement too much thing.

thanks.