I think that there are a lot of things that we can do to improve this site very easily.
1. A Better Community Sandbox
One major inefficiency is the fact that there is a very low level of successful "sandbox to finished product" transitions. There are a lot of abandoned sandbox proposals, many of which I believe could be turned into actual threads. Many sandbox threads are abandoned due to a lack of feedback, as even the creator of the sandbox submission will get bored if there is no feedback.
One way of fixing this is to provide a lot of feedback to sandbox ideas. Even if you only have a very small comment, make a comment anyway because it is important to have a discussion.
Another very important thing to do is to upvote sandbox ideas that you like. When I submit a question to the sandbox, and the question receives very few votes, then I have a high probability of not ever asking that question. By not upvoting a sandbox suggestion, you are helping to kill that suggestion.
One thing to consider is the possibility that we could allow people (other than the original proposer) to move questions from the sandbox into actual questions, if the original proposer hasn't been active for a period of time. If a person submits a question to the sandbox but ends up not posting it, then that whole idea is effectively killed. This is one possible way to revive questions from the sandbox of death.
2. An Idea Bank
We might also be able to make an "idea bank." Basically, the idea bank will be a place where people can go and submit a very brief idea for a thread. Many people don't really know how to write a good question, or they might not have the time. The current sandboxes are designed for fully-formed challenges, not simple and underdeveloped "ideas." The idea bank would be a place were people can donate ideas to the community, so that people can use utilize those ideas in the form of more questions.
For example, say that I had the idea that it would be really cool to create a contest in which the competitor programs a "20 Questions"-type game. I don't know exactly how to implement the idea, so I submit the idea to the idea bank. Later, someone else decides that it would be fun to post a challenge, but they can't make up their mind as to what the challenge is going to be about. That person looks at the idea bank, sees my suggestion, and is able to use it.
3. An Upvote Culture
Upvoting is the number one way to encourage users to remain on the site. If a person receives upvotes, then they feel a sense of appreciation. If a question receives many upvotes, then the user who created that question is more likely to create more questions.
If there is a question that you like, upvote it, even if you do not plan on participating. Also upvote any of the answers that you like.
If there is an answer that you like, then upvote both that answer and the question. It's hard to write a question that gets good answers.
4. Different Types of Questions
So, there's basically two types of questions of this site: code-golf competitions, and competitions in which the goal is very clear and numerically scored. We can expand the variety (and the shear number) of questions if we allow more types of questions.
I think that we should expand the rules to include questions in which there is no way to assign a numerical score, but in which there is still an obvious way to improve yourself. As long as the challenge is difficult, they can serve as challenges to oneself. After all, that's why people participate: for the challenge. A challenge doesn't have to be a competition with a winner.
A second idea is to make a "cooperative challenge." We could find programming puzzles from other locations, and the whole website can work together to beat the best score for the puzzle.
People need feedback. Positive feedback is usually better than negative feedback, but the best type of feedback is constructive feedback. We need to community to come together and help other people become better. Sandbox feedback and other forms of meta participation are very important.
We can start the feedback process with this thread. What do you think about these several proposals?