I think this is an awesome addition to the platform. Looking forward to see what you have put together. My thing with yoast is the passive voice negative marks on posts. They do not explain very well as to a fix for this. I think a great addition to your plugin would be to recommend replacement grammar rather than having the end user rewite the sentence over and over until its not passive voice any more.
Maybe it could identify the text and give 2 or 3 variations of the sentence that could be used .