When the subscription system is used for recurring donations. It would be great to allow for a subscription that has no "Past due". Meaning that if the sub goes into failed or delinquent state that it does not compound the fee. Once the sub is updated/fixed/adjusted it just moves forward and bills the amount it is assigned to.
To have a donor receive a past due amount or go to their account and see that they have a Past Due amount is not very good. In the end it is a donation so if it is not made for whatever reason they should not be required or even asked to pay it.
I nominate "sub_nopastdue=true"