If a union date is coming up for your civ and you know are likely going to be the lesser partner, you could just switch to merchant republic and void the union. I don't like that idea. They're not all necessarily inheritances, are they? I think Republics should still be able to form unions. Maybe they should just be excluded from the ability to become the greater partner.
Also, a question: If a union is a partial one, is it a one time event where benefits are administered and that's that? Or does it result in a special diplomatic status? Like a temporary unbreakable alliance? Vassals are shown in the diplomatic advisor screen and when in diplomacy with the actual vassal, it tells you that the civ is a vassal of another civ. Will partial Personal Unions be shown this way?
Another possible union is the Habsburg Inheritance of 1519. Spain + Austria, really the dominant partnership is up for grabs in that situation. I can't really say whether that situation should result in a complete or partial union, though. Also, an annexation would fudge the potential unions for Austria with Bohemia and Hungary in the case that Spain won the annexation.
Unless union chances for annexed civs can be transferred to the winners of previous unions. Imagine the possibility to recreate the entire Habsburg empire of the 1500s through diplomacy. That's exciting.
Also, in the event of a union between colonial powers, (Iberian Union), does the
annexing partner receive the colonies as well? You know, since they are
projects it isn't entirely clear.
Also, what happens if the human is playing a civ that becomes the lesser partner in a complete union? Is it game over? Does he have an option to switch to the annexing power? Maybe he could have a choice to either switch to the annexing power, end the game, or lose all but one of his cities and get a small army to attempt to fight back the occupiers and retake the nation under a new dynasty? That probably shouldn't happen when an AI is annexed. When cases such as the Spanish Succession result in war, could other cases of unpopular unions result in spawning of barbs in the territory of the annexed? To simulate revolts or the rise of a pretender to the throne?
Also, now that Aragon is featured
beer:
) Spain shouldn't be called Spain until the union with aragon. The civ should be entirely renamed The Castilians. DCNs would be Kingdom of Castille and Leon, then Kingdom of Spain after the union.