This is what I suspected. You are against the best syntax because they are more complex to implement. I think that's coming into the discussion from the wrong direction. First agree on the best syntax, then worry about the implementation.
I also understand the syntax changes will mean a lot of trouble for your plpgsql_check_function() project, but that cannot hold us back, we must aim for the best possible syntax with plpgsql2.
Your work with plpgsql_check_function() btw saved me hundreds of hours of work, when we upgraded from 8.4 a few years ago, many thanks Pavel!