Currently when a user logs into the Office apps using an Office 365 account, they can see the organisation's main site listed within File > Open > Sites - [organisation name]

Please can you add the ability for this list to automatically add any new sites that are made available to the user to this list. e.g. Currently, I have created a new site but the user has to manually enter that URL into the Open or Save boxes to access. This is not ideal (users will just not use the site) so a way to automate these site lists into the office apps would be greatly beneficial.

Within the SharePoint admin centre you could have an option to "Make site visible in Office apps". This, along with site permissions, could control which users can or can't see the site.

This could also make the site appear within the SharePoint mobile app, as this also only displays the main organisation site.