This is now part of the community FAQ.
I suggest that abandoned challenge proposals should be free for others to use as they see fit.
First of all, there are no licence problems with this. All SE posts are licensed under CC BY-SA, which allows modification and redistribution as long as the original source (original author) is attributed. So everything that is holding us back is good manners.
As stated in the question, I think that we're missing out on a lot of good content by ignoring unposted challenges. Hence, by setting up a few rules of courtesy, we should be able to make sue of that content without anyone feeling that their idea has been stolen from them:
- If a challenge proposal was not edited or commented on for a month, you can leave a comment that you would like take over the challenge, get it ready for main and post it.
- If the OP does not reply within two weeks, telling you that they still intend to post the challenge themselves, you are free to proceed with the challenge as you see fit.
- If the proposal is already in the newest sandbox, don't repost, just edit the existing post.
I just made up "a month" and "two weeks", and I'm open to suggestions for better time frames.
There's another reason, why I'm strongly recommending this policy. If we're going through with never retiring the sandbox and deleting posted challenges, then there are only two things left that will make the sandbox grow: generally increased activity on PPCG and challenges that were abandoned and hence never deleted. If we could claim those and still post them, that would make the One Sandbox much more feasible.