Over on PPCG.SE they have a sandbox on meta where questions can get feedback before being posted to the main site. That's vital when designing a programming challenge question that must be precisely specified, but less important on other stack exchange sites where problems with a question can just be pointed out in comments and resolved quickly.
The recent meta question on getting religion questions right is almost a mini sandbox in itself, and makes me wonder if it would be useful for us to have a sandbox too.
We could either have a sandbox meta tag and post questions on meta first before posting them to the main site once feedback has tailored them to be a good fit, or we could use the same approach as PPCG and have a single meta question to which all the answers are potential main site questions waiting for feedback in the comments.
I don't see this as something everyone would use, but it would be there for anyone who has the beginnings of a question but already knows it isn't ready to be posted.