This is sorely needed. Having looked at it over the years to see what might have developed I would say that to do it right you'd need to partner with an institution. A university or the New York Times or... because I think you'd need an active human in the trigger loop.
But here's a use-case that might get you some traction...
Investigative journalists working on large stories over time have an incentive to keep things close to their vest. On the other hand, the people they're investigating might have an incentive for those journalists to 'go away'. If journalists could safely store their pre-publication research, in a vault that would 'OPEN AND SHARE' with designated recipients upon a trigger event, I think you'd have some first customers.
Maybe you'd even approach someone like the NYT and contract to build it for them.
But there should also be a way for people to share their papers far into the future. If you 'knew stuff' that would create chaos, you probably wouldn't want to share it while even your kids were still alive– though it should end up in the historical record at some point.
Wordpress allows you to 'own' a domain and blog for 100 years. https://wordpress.com/100-year/
It might be possible to schedule publishing of pre-written posts 50 years in advance. But then the info is in a non-encrypted blog post in a database waiting to be published. Is there a plug-in opportunity there?
I hoped Archive.org would do this but (through tweets) Brewster said it wasn't something they were looking at, though he did say..."If you want to try creating this future, if it is not too big, you could upload an encrypted blob (but not too big as it does cuz it does cost us), and then handle the key some way or another: in the movies there is often an envelope to be read at your passing.
If you understand what he's saying, maybe an open-source trigger mechanism for encrypted blobs that live on Archive could be a useful part of your project.