Thank you for this suggestion. We don't currently have plans to introduce complete customization into the sidebar. However, we recently announced that we are rolling out a new space sidebar experience in the next few months which offers the ability to customize some features in the sidebar by expanding and collapsing sections. You can read more about it and sign up for the EAP here: https://community.atlassian.com/t5/Confluence-articles/A-brand-new-space-navigation-experience-is-coming-to-Confluence/ba-p/2423015).