<p>I want to separate concerns here. Create and embed all the UI logic for the Custom XML designer, object model, validations etc in to a separate assembly. Then the Package framework should only register the designer information and ask for a UI Service and everything works magically.</p>

<p>This way I don't need to play with the Package framework (Visual Studio Package) assembly, when I need to modify the UI designer.</p>

<p>This question also applies to anything where you have to separate the UI logic from the Skeleton framework that loads it up, like a plugin.</p>

<p>I have several choices a ServiceProvider model, a plugin model or may be other.</p>

<p>Any samples, suggestions for patterns, links are welcome.</p>

<p>Update 1: What I am looking for is a thought such as - "Does Prism (Composite WPF) fit the bill? Has anyone worked on a project/application which does the separation of concerns just like I mentioned above? etc" 
(I am still looking out for answers)</p>

What you're asking about seams very much like the separation of concerns that the
MVC
pattern tries to enforce.
ASP.NET MVC
is already out there with a
preview 5
.
It's mainly for web but I think they are planning on using it also for WinForms, but I'm not sure.