I don't think it's possible to come up with fixed rules that don't fail, which is why I like gofmt's approach of keeping the general structure of the input code, instead of overriding it with rules.
So my rule would be don't change the number of lines. If a construct is in one line, keep it in one line (but tidy up spaces after commas, etc.) If a construct is in multiple lines, make sure the indentation is even, but don't unwrap them.