I have seen quite a few "sandboxes" on meta SE's and it seems like a brilliant idea. The problem I have, and I think other people may have too, is that the questions can sometimes be vague/confusing, too broad, badly put, etc. and a lot of the time, those questions get downvoted a lot and closed and eventually deleted (as they should).
The "sandbox" would be a place where questions which may be bad can exist without threat of the question getting downvoted into oblivion and the person getting punished for a bad question and said questions would be instead criticised for how to improve them (rather than get answered), and this way bad questions can be made into good questions, which can then get asked on the main site. Yes, this may induce spam and misbehaviour but they can get removed. Note that the reasons for closing and ultimately deleting a question in the sandbox would not be because of the question's quality, but rather the asker's lack of responding to the community suggesting ways to improve it and also the previously mentioned reasons of spamming and deliberate misbehaviour.
The low-quality questions can potentially be turned into high-quality ones and it means less low-quality questions on the main site.