There are pages on e621 that can't use the full width of the screen and locks all the content to the left, which looks bad on normal and big screen.
It would be great if those content, limited in width, could be centered. It would looks nicer regardless of the screen.
The affected page that disturbs me the most is the user settings page.
The login page is also quite affected by that.
Other candidate pages I've seen are : static themes, site map (in 'More'), keyboard shortcuts, potentially the Help home page (more subject to discussion) and maybe Terms of service.