The killer feature for me would be able to "expand" method implementations at a call site e.g.

    class UserController
       def create
         ...
         UserMailer.send_invitation(user) # expanded line
           def send_invitation(user) # implementation from UserMailer
             ...
           end
       end
    end

This way you could unfold the entire call stack in place i.e. you can see all the code you're working on at once.