Ivan may not be perfect but I think he would be very interesting as a civ leader, I don't see any reason why they should stick with Catherine for Civ VI, or Elizabeth for England.
IVAN the terrible de facto created the Russian Empire, he expanded it eastward, captured Kazan (Khazan) and further east into Siberia.
He organized the army, defeated countless Tartar (and other) tribes dominating to this point the vast landmass east of Moscow.
He was a despot and a fanatic Orthodox Christian at the same time.
Ruthless, Imperialistic, Spiritual, Militarily-Scientific.
He created the infamous Oprichniki, being psychotic about spies ordered countless innocent people tortured and murdered, then would try and repent by mutilating himself.
A truly incredibly interesting character and a great Great Leader pick.
BY THE WAY, I'VE SUGGESTED HIM BEING ONE OF THE GREAT LEADERS OF RUSSIA SEVERAL TIMES BEFORE IN THE IDEAS AND SUGGESTIONS FORUM.
but hey, nobody cares what daft writes, until others re-discover it later and take the credit for themselves, cool.
Cuauhtemoc for Azteca.
Cetswhayo for Zulu
William (the conqueror) for Normandy (France)
Tamerlane (Timur) and his Timurid Empire
Native America: Tecumseh, Geronimo
Iroquois: Dekanawida
Sioux: Crazy Horse
England: Oliver Cromwell
Russia: Nicholas II
Rome: Hadrian, Marcus Aurelius, Octavian, Claudius
Vikings (Norse/Norway): Harald Hardrada
Greece: Agamemnon, Pericles (of Athens)
Egypt: Akhenaten
Basil is a great one!
By and large almost anyone other then Theodora or Justinian would be nice. (For onlookers: They aren't really Byzantines IMO- Byzantium only grows into a distinct entity after reign of Heraclius and his reforms making Greek the language of government. That's the point of no return in terms of cultural evolution from E. Roman into something different.)
I happen to love Nikephorus though, but I also think its high time we lived in a world that had
"Conan" (or as close as wel can come to it) as a leader in civ. 
Conan was/is a Celtic mythical hero, no?
so, you never know.
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