Maybe it has been suggested before? Perhaps it could fit in with the caravan arc.
If your civilization is at peace with another race, they could send an ambassador once your fortress is deemed worthy.
The ambassador would be a permanent, non-laboring citizen of the fortress. Much like nobles, they would require decent rooms but would not mandate anything. Once they are settled in, it would be possible to send your mayor discuss with them, and change the goods you require form their caravan ( provided the caravan is not coming soon) even after the liaison is gone. It would also be possible to send gifts to their monarch or discuss peace treaties should you end killing one too many caravans. The elf could warn you when you are about to reach your tree quota ( Not that anyone cares).
In their free time, the ambassadors would hang around the meeting hall, waste booze and food, or remain in their quarters. Human and dwarven ambassadors could also spend some free time sparring with your military.
It would also be cool if these ambassadors could share some of its race specialties, such as humans teaching your dwarves how to use exotic weapons, elves teaching you how to make wooden armor, and dwarven ambassador from other civs could teach you how to make tongas and skirts, or things that is not available from the start.
Of course, killing the ambassador wouldn't be very appreciated.