I think one venture possibility in this area would be some sort of add-on to analyse the data coming from other educational software and organize the information for teachers. This might manifest itself as a weekly or monthly report directed to teachers with the names of students and the types of problems they’re having under different headings like, perhaps, “grammar and spelling”, “essay structure”, “transitions”, etc. This would provide a kind of formative assessment that would assist the teacher in personalizing teaching. Of course, there would still be greyer or more subtle aspects of writing, especially in older grades, that may not be easily identified by computer programs, and would still need to be identified by the human teacher, but it could help a lot with more common easily identified errors in writing.
As an aside, I don’t know why there is so little K-3 in this survey; I downloaded “Letter School” for my 4 year-old after reading about it in your OER and the app store had countless suggestions for me of other similar apps. There is, of course, the question of how much screen time a child should get, though.