A 'Reviewer notes' system could be VERY useful. What often happens is that reviewers will put some partial effort into reviewing an article, and the give up after hitting a roadblock and being unsure. An ability to write a comment on the talk page that is automatically flagged by the toolbar if another reviewer loads the page would be ideal.
Reviewers could write a comment in a field in the Page Curation toolbar, which would then copy this note to the talk page and create a section with a unique header "New page reviewers' comments" (or similar). The page curation tool would scan for a section with this exact header whenever the page was loaded up and automatically notify future reviewers that another reviewer left a comment, this ensures that whenever another reviewer looks at the page, they immediately know that someone else already left a note on the talk page. While the talk page can currently be used in this manner in the same way, reviewers won't always check this for new articles, as there are rarely content comments on new articles. Reviewers could then comment via the talk page directly (under the same header). I think this would be a very useful feature.
This task should be combined with T207443, (i.e. they would both use the same templated heading, and those comments would also be flagged)