I would argue that we don't really need to increase traffic to Worldbuilding. Worldbuilding SE is getting a fair amount of traffic. It's certainly as active as, if not more so than, both Physics and Aviation.
The community that decides whether they want a question sandbox, and where it is to be hosted, should be the community in question. Nothing is gained by one site imposing a standard onto other sites. Imagine if, say, Mi Yodeya was to host a sandbox for us! What are the odds that people posting there would get good advice on what makes a good Worldbuilding SE question, beyond the absolute basics like, say, being able to tell what the question is? Now, why would it be any different if it's the other way around?
Also, for example, the issue of whether to create a sandbox was raised on Writing Meta not all that long ago. The answer from the community there, judging by the votes, was a resounding NO.
Each site needs to determine on its own whether having a given fraction of the questions posted put on hold is even a problem. To me, putting a question on hold is not really a big deal, if it's warranted; it keeps potentially many more people from wasting their time. If the site's community feels it's a problem having the number of questions closed that they get, then that community needs to decide how to deal with that. A site that aims for high quality questions is pretty naturally going to put on hold a larger percentage of the questions that do get posted.
We should not be imposing our ideas, let alone standards on what makes a good question, on other communities in the network.
However, nothing prevents you from suggesting the idea of a per-site question sandbox on each site's meta, if you think it is worthwhile.