Teaching is difficult enough from a personal performance and logistical standpoint that there is low hanging fruit in simply helping teachers be their best professional self.

For example: I have approx. 200 high school level students. If I assign them to write one page I have just given myself 200 pages of assessment to complete. Assessing writing is more cognitively demanding than simply reading 200 pages, so if I can read and provide feedback on each page in 3 minutes - working non-stop - I have 10 hours of work. I have 5 hours of prep time per week which is primarily used for preparing the 7 hours a day I spend actively in class with students. Given that we don't complete just one assignment per week - you can see the logistical difficulty.

I teach english. There are commenting/notemaking tools in almost every online word processing toolset, but none of them are streamlined for assessing writing (i.e. providing rubric based feedback, rapidly annotating with custom or pre-created notes, allowing for layered annotations to identify different structural features of writing). There's opportunity in figuring out what teachers already do and slash the time required to do it.

Unfortunately (1) it's fragmented into lots of sites ... but most importantly, (2) it's low quality, and typically not adjusted to your particular year (From year to year the level of students varies a lot, and some need easier or more difficult content). On top of that, some years you have 50h with the students, other years 60h (depending on holidays, breaks, outings, interruptions). So that "package" would need some input parameters to achieve differentiation, acceleration, etc.

I often find it easier to spend 2h creating my content from scratch than adapt somebody else's (This can mean 20h - 30h of preparation in a week when you are teaching something for the first time, on top of teaching, supervising, after-school activities, etc)