Area 51 is what it is, and it is not perfect, nor likely to be updated anytime soon.
We have an FAQ but it does not really help to define what a viable or successful site might look like. There have been some recent discussions 1, 2, 3 about the difficulty of defining what is likely to succeed.
What kind of inclusive statement, would work to help site proposers pick and define proposals with the highest chance of success? The answer(s) should be inclusive, (do this) not exclusive (don't do that). We want to encourage participation in a positive, welcoming and helpful statement.