If subscription is available there is a licensing model needed too!! If not, someone can pay one subscription fee and own a copy of the module for ever. Also vendor must have the chance to decide if they want to sell their modules per subscription.
Absolutely. We will definitely support both options - buy or subscribe or both, depending on what vendor decides. And you're right - the trickiest part is to control licensing on subscription-based extensions. This is what we are working on now in Trident and when we get it right, we'll bring it to DolphinPro. In a nutshell - extensions are downloaded directly from within the site-admin and Boonex tracks subscriptions status, deactivating the ones that go unpaid (with prior warnings, keeping the see more data untouched).