What about the good but abandoned proposals?
Ask for permission, adopt and post all posts that have a score > 1 and no activity for a month.
Give people notice first
Before VTDing a post, comment this:
Canned message to comment to to-be-deleted posts:
This sandbox post has had little activity in a while and little positive reception from the community. Please improve / edit it or delete it to help us [clean up the sandbox.](https://codegolf.meta.stackexchange.com/q/12792)
Whatever action is taken, if it isn't "No action" then it has the potential to be incredibly disruptive. The same principle applies as with big retagging projects: if everything is tackled at once then it floods the first page with trivial changes, burying new content.
If 50 posts are edited as a result of their owners responding to warnings about possible deletion then all recent sandboxed posts will be pushed back two pages, and the amount of "new" material will be so much that some of the ideas which people are trying to save will not be reviewed.
In short: doing this badly will exacerbate the problem it's trying to solve. Do not rush.