To harmonize the data model as much as possible it is a good practice to define the softtypes on site level.
But when you have accomplished the challenge to harmonize on attributes itself the even bigger challenge is already waiting in terms of constraints and enumerations.
It would be an great enhancement if the constraints could be set on the organizational level. Currently this is OOTB only possible via additional subtypes
To complete this requirement it would be great as well to control the visibility for attribute on this level as well. Currently this is OOTB only possible via profiles which are linked to a user and not to an object owning organization.
Both would enable great flexibility when required while streamlining the data model and will reduce administration and complexity