It's been asked before, it's been rejected before and now I'm bringing it up again.
I have a love/hate relationship with the Sandbox. I've voiced complaints before but I now have an idea that might work.
The Sandbox is quite a good idea, despite having flaws. One of the flaws that we can easily address is its size. As if posting this, it has 1700 proposals. This is great. Unless you happen to use the iOS app (no idea about android) where it slows down massively, making reviewing posts difficult.
In a (apparently) unrelated side-note, the Sandbox is approaching 2000 answers. What a milestone. Almost makes you think that we should give it some form of reward...
I propose that we combat the problem in the first paragraph with the solution in the second. When the Sandbox reaches 2000 posts, we lock it and allow active challenges there to be transferred across to... a new Sandbox! I'm sure that the Sandbox would be happy to retire from that question (not to mention the fact that it will get another 3 gold badges).
I also suggest that we extend this into the future, if this works, which I see no reason why it shouldn't. When the next sandbox hits 2000 answers, we lock it and create a new one.
An advantage that waiting for 300 more challenges has, don't forget, is that it will allow us to slowly work down the number of questions being posted there, especially if we begin the new Sandbox as the current one nears 1950 to allow for people to divert away from it.
As an extra bonus, it will probably (for the moment) solve this question as people are more willing to look at a post when it has less answers.
Now, I have been presenting this as something that is perfect, that can't go wrong. But I'm going to do a Geobits and point out what's wrong with this.
To kick off, let's look at the reason why we have a giant Sandbox. It's because we had this situation before and it wasn't working. To quote Martin Ender, it was causing more problems that it was solving. If we reintroduced that, wouldn't it cause the same problems?
Also, how can we make sure that people won't keep posting in the current Sandbox as we are retiring it? The ideal thing would be a lock where people can edit, vote and comment but can't answer. But of course this doesn't exist and SE won't make an exception for us.
Finally, have you seen the proposed design for the site? If not, check it out here. In my opinion, it is a brilliant idea. But, when looking at it, I noticed a "Propose a Challenge" button that presumably takes you to the Sandbox. You're beginning to see the problem here, aren't you? If we constantly change the Sandbox, the button is going to have to change, as is the "Featured on Meta" link. Mommy SE will not be happy having to update things like this every so often.
In short, what do you think? I will post a "Yes" and a "No" answer but if you want to elaborate, feel free to post a separate answer.