Use by date
It's interesting that India, though they haven't raised the possibility as loudly as China are also considering digital currencies with expiry dates: https://m.rbi.org.in//Scripts/PublicationReportDetails.aspx?UrlPage=&ID=1218
It's not a new concept but the technology makes the implementation much easier. This is known as Gesell economics, where accumulation of wealth is divorced from the currency used for everyday transactions.
Also, note the concept of tokens with spending restrictions. They give the example of agricultural stimulus tokens only being able to purchase agricultural supplies. It's strange to think of a future in which your employer might pay you a salary where your budget for food, energy and discretionary purchases (some of which you may be permitted to put into savings) is already allocated for you.