Message self-destruct setting
A self-destruct action would be like a snooze action, except that instead of resurfacing at a particular time, the email would automatically move to the Trash or Archive (per a preference setting).
This would be very useful for limited-time offers, newsletters, etc., that might have a limited shelf life. If an offer from a retailer is going to expire in two weeks, I might want to keep the email around in case I decide to shop at the store before then. But I don't need a bunch of expired offers littering my email.
Even better: If the app could use heuristics to try to identify such messages and recommend self-destruct times (but just as suggestions somehow, in case it's a false match).