This topic has come up before on this forum. We don’t think a metadata format should be standardized as part of CommonMark (see especially Metadata in documents). As you probably know, there are already a number of implementations that allow the use of YAML metadata. See for example lcmark and pandoc. There’s quite a bit of discussion on the pandoc bug tracker about various complexities that have come up in the real world; you may want to look there as you work out your ideas.
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