I guess this falls under UI? I'm ADHD and it would be SUPER helpful to have a feature where I can dim the screen except for an area I'm currently reading. I read a lot of professional articles and when I go to write a note I completely lose my place. It could be just a static section and as I scroll the section stays the same so I'm effectively moving my page through it. I can cope if I print out the pages, but articles are frequently 12+ pages each and I'd rather save the paper and be able to "filter" digitally.
If you're feeling really fancy, maybe make it so the size and location of the filter can be changed.
Concept example attached.