[development] module for taxonomies in an external database

Is there any module that allows me to have just some taxonomies stored in an external database not necessarily using the same tables as in standard drupal? If not, current taxonomy APIs are powerful enough to allow such a development? I am asking because taxonomies lower expressive power if compared to a relational database or to onthologies. I would like to build a module that allows me to define a taxonomy using metadata which describe how some knowledge stored in a db is projected on to a taxonomy. Thanks, Davide