The coupon system is going to be refactored before much longer. This will open it up to more possibilities, right now the coupon system is in a feature freeze due to the necessity of refactoring.
There is a package option planned called package switching where a customer may order 1 particular package, and then it will switch after x renewals to a separate package. This could be used to offer a free domain registration, that automatically changes after order so that it costs something when it renews.
The domain registrar plugin system is going to improve domain registrations overall by creating a pricing grid for TLDs (easier pricing than on the package) and consolidating registrar features. So, there's a lot that is going to be happening in terms of domains, and we appreciate your feedback.