I'm not sure if this is already possible, but it would be nice to have vouchers that have no preset expiry date, but can expire after a set time from the first login. This would be useful for say, a front desk of a hotel could generate a bunch of vouchers and keep them in a drawer. When a customer books a room for a day, the customer can get the voucher, then login to the WiFi from their device. After that inital login, the expiry date is that current time plus 24 hours.
This would mean that vouchers can be generated in batches on a regular basis rather than on-demand, which would be less work for staff.