As noted in other posts the areas with the greatest gaps are K-3, Provides Assessment and Psychomotor skills along with Manage Learner Profile, Provide encouragement, (which I found odd as many of the apps seem to do this) and Evaluate Learner Progress. I think a gap appears on your data when in fact I know there are a large number of apps available for Pre-k-3 reading and writing skill development. These skills tend to be developed together in the early years.
Writing for K-3 involves so many different skills starting with learning the alphabet (sounds and shape for psychomotor skill development) and learning to read basic words leading to early sentence structure. Many apps provide both reading prediction and writing prediction technology. Apps for digital storytelling develop a child’s imagination and support early writing skill development. These apps also provide encouragement for the learner and can be used to evaluate learner progress of content output.
The K-3 gap would not be an area I would pursue as I think it is fairly saturated. I would pursue personal writing technology for grades 4-7 trying to find ways to both remediate and motivate writers while providing management of learner profile and assessment. There are many reading programs available to do this, but not as many writing programs. In one of my other courses we developed a Community of Writers website to address try to address this age group and these target areas. The Community of Writers (COW) provided lessons for skill development, story ideas and starters, and an opportunity to have peers provide feedback and an audience for personal writing. Teachers could assess from the site, but the focus was more for peer assessment and collaborative writing. I would pursue this area further finding ways add assessment and learner management into the website.
Personalized writing will become more technology dependent than it already is. I think we need to encourage creativity and imagination in writing so that students, especially young students, do not rely on predictive writing technology to write for them. Though this technology supports remediation I have watched many students who do not need it rely heavily on it, much like spell check!