Regarding legendaries; a dwarf who is skilled and capable of tremendous masterpieces should be attracted to the greatest centers of dwarven civilization, much how great artists tend to collect in Paris. While a fortress remains a dingy outpost with a dozen -larch beds- stacked in a corner of a storeroom, it should be nearly impossible to keep a legendary dwarf from wanting to go see the world. What's more, the dwarven king himself might request that legendary dwarf to come make him *Crystal Glass Goblets* or engrave his palace, drawing the dwarf back to the mountainhomes. This would make legendary dwarves both a blessing and a curse as the player uses all the fine products of the dwarf's labor to improve the fortress, but at the same time must ensure his legendary craftsdwarf has a big apartment. To be honest I'd like to see them added to the Noble's screen.
Of course once your fortress becomes home to the king, things change. Legendary dwarves would want to remain where their works can better glorify the sovereign. The problem might then shift as the lower classes, unable to afford bedrooms covered in masterwork engravings and eat gourmet *dwarven syrup roasts*, are driven out! This might well result in a whole new definition of 'fun' as we realize exactly why our starting seven left in the first place and create a natural 'life cycle' for dwarven cities; foundation, growth, maturity, apogee, decay, abandonment. Intelligent management of dwarves would be required to keep your fortress living indefinately; every dwarf a potential legendary, every legendary a snooty rich elitist, every rich elitist another curse on the common dwarf. Heck, kind of explains nobles. Retired legendaries.
Naturally, the desire to migrate could be controlled by a personality tag and influenced if the dwarf was born locally, has local relations, and so on. An unmarried legendary dwarf who originally migrated in, had a mood, and is now a miraculous gemcutter likely does not want to sit around your fort as long as a local-born, married miner who toiled and toiled to acheive his legendary status by carving out half the mountain.