Proposal: Mathematical ideas
We already have
How many more stackexchange sites about math do we need?
I have a hard time seeing how this site could be successful. It is true that math.SE and MO rather discourage the presentation of "mathematical ideas." However, this is not for some arbitrary reason, but quite simply as this type of "question" does not seem well-suited for the format, as often it is not an actual answerable question.
To put it differently: a question suitable for an SE site with the given subject, should be on ontopic on MO or math.SE. If it is not ontopic there, it likely will not work in the SE format, which--circling back--is why they are not ontopic there.