What about the data add ons rolling over? That's what Slingshot do and it seems to be very fair. Pay $50 for 50GB and only use 25? Then next month you still have the extra 25GB on your plan to use, but the base plan doesn't roll over.
That's fairer for people as they're using the extra data they've paid for, and fair for the ISP as they're not rolling over larger and larger amounts thanks to the base plan.