@uMathieu Actually, turns out, I would like to have more than one container. I would also like to access some attributes of elements inside the template instance and changed them.
Concretely, a common node in my case has a type, a name, content container and port containers. I am not shying away from making it into a custom control, and from what i gathered here UI builder and custom elements I guess I could make make those attributes accessible easily enough. But what about these two containers? Can I expose them (VisualElements) as properties and then adress them from XAML with for example <Instance.PortContainer> or <Instance.ContentContainer> similarly to how it would work in WPF?