Basically how the field "tag" now work, but "improved" :-)

Must have

• Vocabularies (pre-defined) of terms
• Centralised management (an overview in the panel where you can add/edit/delete terms in a certain vocabulary and those changes are reflected into the linked content)
• Multilingual
• Terms can be sorted inside a vocabulary (order)
• Plays nice with API such as filter() or sort()
• Freetagging: Terms can still be added from the creation of a page, but then they are "automagically" added to the vocabulary — will be a nice mindbreaker to find a nice UI for this to work multilingual

Nice to have

• Hierarchy between terms are possible
• Terms can be extended with fields

Upon writing this, it comes to mind that some kind of "relation" or "reference"-field, combined with some UI in the panel for the referencing part might solve most of the above?