I'd love to see a tool to help us write good quality SEO articles.  At the moment we write our articles in a private Wordpress site and use Yoast SEO to get our SEO right and as close to as we can.  It's just a great tool and as a new subscriber to Sendible I miss this, I'm not saying integrate with Yoast (but that would be great) but just some visual indicators to make sure we get the SEO basics right.