The Iberian Union has always interested me, and I like to wonder what would have happened had it been permanent, or, at least, far longer lasting.
With no POD before 1550, and not after 1700, make the Iberian Union able to survive until at least 1800 and speculate as to how this would change history elsewhere.
Bonus points if you get them to unify.
Mega super bonus points for having them unify and then turning them into a republic instead of a kingdom