Is it possible to create a MVC root application (Portal with masterpages and themes) and add a couple of WebForms based subprojects (we already have an existing WebForms application that we would like to integrate into the Portal)?
How would you centralize navigation (sitemaps, url routing)?
How would you share the masterpages?
How would you refer to resources (~ issues etc.)?
Combining web forms with MVC is entirely possible. See this blog post by Scott Hanselman for an introduction.