@Lisa_Catto, As just a possible workaround if/until that becomes available (and I think sub-divisions of organizations have been shown in Asana future concept videos), have you considered making some Teams Hidden so as to un-clutter some users’ sidebars? Also, I know there’s the concept of Divisions (Adding new teams within a premium Division Team - #4 by Wim) which seems related, but I don’t think that will address your need.
Larry