Some of my ideas:
Reworked Migration
Migration, currently, is a bit silly. Dwarves are sent from the Parent Civilization to populate your fortress, not caring whether they like it there or not, and with no way to leave again if they don't. Transitive populations provide an alternative. Dwarves in the pool of drunkards could decide they like your settlement enough to decide to stay. Additionally, you could finally impose citizenship requirements on migrants, and only allow the cream of the crop (Disclaimer: Zarathustra30 nor associates do not endorse Dwarven Eugenics in any way, shape, or form).
Inebriated brawling
Speaking of Drunk Tanks, some of the non-dwarf taverngoers will not be able to hold their liquor. They will become unruly and need to be detained by your fortress guard. From there, a wise overseer would be sure to carry out justice to prevent it from happening again. Imprisoning non-residents is far more productive than imprisoning masterwork crafters for not making enough crowns.
Religion
Sometimes a Religious Fanatic or a High Priest will come to your fortress, spreading the word of Knurd, God of Alcohol Dependency. A few dwarves may convert, and a noble Priest may be anointed (complete with demands for a temple). If all goes well, religious pilgrims will start to arrive and pay a tithe for the "maintenance" of the temple. If you are large enough, you may be host to multiple religions, which may cause Fun in the dining rooms.
Plague
With all of those filthy immigrants (racism not intended), a disease is bound to turn up. One dwarf in twenty may show up with Swine Flu. If not quickly quarantined, the plague will spread. If things go disastrously wrong, the transient population could flee, spreading the disease to other sites. It would be very Fun to be on receiving end of a swarm of plague refugees still in the incubation stage.
Feel free to expand.