Lots of criticism here about the approach, but I rather like it! I think if you can execute well on this app, it'll be a huge hit! It might be a shortcut to good writing, but who doesn't like a shortcut? Also I do believe that if you produce good writing with the help of an app, sooner or later you'll pick up on the little tricks your app frequently suggest, and you'll learn and actually become a better writer!

Actually I think it might be the best way to learn, because it'll teach you to write well in contexts that are relevant to you. Someone suggested reading fiction from good authors, but that'll teach you to write good fiction, not engineering emails. The skills might be transferable to some degree, since it is rather similar, but it's not the same thing!

Kudos on this idea, and I'd definitely pay for it if it works well enough!