One thing I haven't seen used much is a license that effectively voluntarily shortens a copyright term. There's a lot of people here who might want to license s

One thing I haven't seen used much is a license that effectively voluntarily shortens a copyright term. There's a lot of people here who might want to license something either commercially or as Creative Commons, rather than dedicating it to the public domain immediately. But why not add text to the license that dedicates it at a future date, corresponding to however long you feel the copyright should last (say, 30 years)?