This occurred to me when checking Isabella in the new leaders topic, but I think it fits more thematically with Spain's treasure fleets: a minor boost to the UA that adds extra golden age points for navigation or sea-based historic achievements (we know world's circumnavegation is one of these, and both this and the discovery of a new world are credited to Spain -
yes, Norway disagrees, but nobody knew of Vindand until waaay after -, plus relevant contributions to marine corps, amphibious warfare, submarines,... ).
Spain, (and specially both Philip II and Isabella as leader), is quite a target for religion-based loyalty bonus-malus.
Already commented, Barbarossa should use same combat bonuses for free cities than for city states, altough at the cost of dealing with loyalty issues. (This is, his cities would not be the most loyal ones, but he would be able to recover them by force... plus get some cities from his neibourghs as well).
Persian Satraphea could get linked to governors (extra trade route bonus for governor cities?), if not overlaping too much with a potential Ottoman trait.
America seems a potential target for minor bonuses on government district and loyalty by culture, but nothing game-changing. Victoria may be also a candidate for loyalty bonuses based on cultural, military or scientific power (victorian fondness on "the empire" which set the "civilized" standard)
Saladin, as a paladin of his faith, could have a boost on potential achievements related to defending its religion.
Greece's leaders would fit as well for a minor and differentiated golden age points boost: Pericles/Athens based on culture and alliances and Gorgo/Sparta based on military achievements.
China's Dynastic Cicles may be used to potentiate or tweak golden/dark ages in some way (reflecting it has had ups and lows, but it has always remained a remarkable power, it could avoid some of the challenges introduced by "rise and fall" (while still benefiting from golden ages). -> Maybe shorter dark ages and longer golden ages, but no heroic ages.
As comented Cleo (Economy), Gilgamesh (military), Peter (science) may be given some alliance bonus.
Some crazy ones
* Strong governors (maybe free military trait) but loyalty issues in their cities if different-leveled for Macedon (companions-succesor states?)
* Japan golden / dark ages impacting greatly its alliegance bonuses (isolationism / commerce switch).
* Kongo's Mbembe Nzinga should get even more crazy bonuses by joining religious alliegances (and will hate you if you reject signing this tipe of alliance).
* Norway could "raid" golden age points from other civs (this is, geting raided by vikings makes you more likely to get a Dark Age, and at the same time makes them more likely to get a Golden Age).
* Rome's governors are more powerful, but become a big loyalty issue entering a dark age (so much you are likely to not be able to use them or expose to immedate civil war (city loss as free city))