This is a list which merges the leaders from SadoMacho's pack with the proposed new ideas:
....
Rome: Julius, Augustus, Justinian, Mussolini
Japan: Tokugawa, Hirohito
....
Vikings: Ragnar, Harald Hardraade
For Japan, I think we should add:
- The Meiji emperor (Mutsuhito), to represent the Meiji restoration and creation of a strong, modern, and industrialised Japan, letting his country emerge as one of the great powers on the world stage.
He could be used to cover all 19th century (even if his reign started in 1867).
- The Shōwa emperor (Hirohito)...
This is more tricky.
The Shōwa period was the longest reign of all Japanese emperors and japan went from ultra-nationalism with WWII, to modern democracy and strongly pacifist outline... all under the same emperor (Hirohito).
Funny enough Shōwa means "period of enlightened peace"... considering the events of WWII it's ironic.
However he was extremely influential in creating contemporary Japan and could be used to cover all 20th century (ascension date 1926).
Else we could use the current emperor... but his outlook is minor compared to the others.
Rome... leaders after Justinian maybe not really necessary.
Not always Rome survive or respawn in modern times.
For all middle age and renaissance there is not one single leader that can represent Rome or all Italy.
Maybe for this period we could use one of the many Popes like Pope Julius II (sistine chapel and a few wars) or one of the Borgia.
Else Lorenzo de' Medici could be a good candidate to represent a mercantilistic country.
For more modern times a leader representing the unification of Italy could be Camillo Benso, count of Cavour, that was the brain behind the return of Italy to independence (1861).
Else Victor Emmanuel, 1st king of Italy, could do.
After that Mussolini is a candidate for 20th century, and late try to create a colonial empire.
However in a longer historical view Mussolini didn't do so much new, mostly intensified processes already existing in the country.
Maybe it would be better to have somebody representing democratic Italy (from 1946), but no single name is big enough to emerge over the others.
Vikings:
17th century Gustavus Adolphus king of Sweden.
Under him Sweden emerged as a European Great Power controlling all scandinavia and baltic region.
20th century I would suggest Swedish Prime Minister Olof Palme to represent social democratic scandinavia.
the current Swedish king Carl XVI Gustaf (since 1973) would be fine too.