I am not sure what you mean with new page? From a content area you can only reference existing pages. Todo so you could have override SetDefaultValues in your model class and do something like:

public override void SetDefaultValues(ContentType contentType)
{
     base.SetDefaultValues(contentType);
     MainContentArea = new ContentArea();
     MainContentArea.Add(ServiceLocator.Current.GetInstance<IContentLoader>().Get<StartPage>(ContentReference.StartPage));
}

Note that the Add method is obsolete as of EPiServer 7.5, which means you need to add a ContentAreaItem to the ContentArea's Items property instead.