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A neat feature would be to have the coupon code pane inactivated when there are no active coupons. This is really a nice-to-have feature and not a must, but the checkout panes get cluttered quite easily when using shipping modules, several payment methods etc. so anything that can reduce clutter when it's not needed would be useful.
The idea is basically that if the are no active coupons, the coupon fieldset should not be visible in checkout