Add better content formatting capabilities to the basic content editor
The WYSIWYG-style editing that the content editor provides feels dated, and the formatting possibilities are very limited. A more modern approach would be very welcomed, and would allow us to get a lot more flexibility/usage out of the built-in content creator.
A great example of a solid content editing/formatting system is the one used in Atlassian's Confluence. Anything close to that would be a huge improvement.
John Hope commented
Froala is also a good editor