Have you considered introducing the option for metered content? What I mean is that sites could allow visitors to view a certain number of articles each month for free, before being forced to sign up for a subscription?
This model - which also allows search engines to view and index content behind the paywall - seems to be how many subscriptions sites are going. Wordpress and Drupal both have options for this... but there don't appear to be any components for Joomla.
I'd love to see this, and I think it would put Membership Pro way ahead of the curve.
(My company is going to be transitioning to this business model, and I'd love to be able to do this with Joomla, rather than a whole new CMS...)