I hate to be the bearer of bad news, but this feature request has been declined.
The default files tab greatly limits the architecture modelling for upgrades from classic SharePoint structures. No thought has been put into declining this, why wouldn't you allow users to specify a default library location while creating a team rather than forcing an ugly structure behind the scenes and limiting upgradability?
Has been a major impact to the user experience for several of my clients.
This feature request is working its way through the backlog queue. No new details are available yet.