I'd like to be able to add on 3 (or any number of my choosing) months onto an annual subscription. So if a customer pays annually they get 3 months free.
Or, alternatively, I'd like to be able to give someone a coupon code and the coupon will add on additional months to the annual subscription.
Essentially it would add a free trial to the end of the subscription.
For things like Black Friday or special events I think this would be very useful.