none that have existed!
...
(unplayable) Great Barbarian: especially with games with large/huge maps, some continents can be populated with many barbarian cities.
…If more than, say, 4 barbarian cities are created in a continent, and your civ is in contact with one of those cities, a diplomacy screen exists with a Great Barbarian
…no civic proposal, no goods proposal, no treaty except cease fire (no Peace Treaties)
…can only be bribed with gold or a military technology (no growth, no economic or scientific) or military units or cities
…can propose slaves (workers), maps and cities for sale and at high price (for just giving the feeling that it is better to destroy them than deal with them!).
… they can also propose skilled explorers for hire. (Explorers are modified for being invisible to Barbarians, as otherwise they are too easily killed and pointless, especially in large maps where Barbarians are quickly way too strong for Explorers)
…The Great Barbarian can NOT be contacted at will via diplomacy screen, he/she (walkiry) contacts you when he/she feels the needs, and only when you have a unit against a Barbarian cities (barbarian units in the wild are just illiterates…

. Especially useful on large/huge maps and marathon play!!
OPEN leader: a generic leader, available in Custom Games only, with Sid or Soren’s face or a female pretty-but-tough face (!), and you choose the associations (philo, aggress, fin, cult, etc…

AND the starting 2 tecs.
…Other specifications are taken randomly from another civ (special building/unit, color) at game start, or can be also choosen by player (in this case, take the whole civ spec: special unit+color+civ name+city names).
…For choosing this customized leader, game difficulty can NOT be less than Monarch (or perhaps Emperor)! This allows what-if games, and test the highest challenges for those who can not do it with a normal game (99% of us??...)