There are many possible knobs on such a tool that could be wonderfully useful. I'd love to press a button that provides critical feedback: marking for errors or suggesting improvements for style (grammar, better word choice, useful idioms, etc).

It'd also be great if my sentence was scored for 'natural-ness' -- how close was it to native speech? If this could be done regionally, that'd be better still (e.g. Spanish as spoken in Spain vs Mexico vs Argentina, etc).

I'd also love to know what age level am I speaking at. A child? A teen? An adult? A more educated person? A professional author? Am I being ambiguous? Too direct?

What is the tone I am using? Friendly? Demanding? Too formal? Culturally clueless? Improper to someone older or to the opposite sex?

In fact, these same feedbacks could be really valuable even in your own native language. Learning how to communicate more clearly, precisely, positively, professionally, respectfully, literately, or even in a different style would be great practice to improve a person's basic communication skills in any language.