The following is based on the hypothesis that the description of the pantheons in Heaven's culture and gods page is correct (I think so).
In the mythic age, Zeus is the only one to have Hera, it means that his two brothers have the same choice in that age : Hephaistos or Artemis.
I made the calculations of all possible intersections between two greek pantheons from two different main gods to see what is the expectation for the number of minor gods shared by two players. Here are the results:
(note that the total number of combinations is always 8*8 that is 64)
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Zeus and Poseidon or Zeus and Hades :
27 combinations with 0 common minor god
27 combinations with 1 common minor god
9 combinations with 2 common minor gods
1 combination with 3 common minor gods
the expectation is : 0.75 common minor god
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Poseidon and Hades
18 combinations with 0 common minor god
30 combinations with 1 common minor god
14 combinations of 2 common minor gods
2 combinations of 3 common minor gods
the expectation is 1.0 common minor god.
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Therefore, Zeus is somewhat more unique than his brothers as his pantheons have less intersections with it's brothers pantheons than these two have between them.
But, the expected number of common minor gods is lower than I tought, even between Hades and Poseidon with the same choice in age 4.
Of course, those figures are based on the assumption that minor gods will have equal popularity which will certainly not be the case and the exact expectations might vary greatly with these if the game is not well balanced. The better the balance is, the closer we will be from those theoritical expectations and of course, if the option random pantheon exists, it will provide exactly those expectations.
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Comment : these expectations might be calculated in a much simpler way without all the details shown above, but I wanted those details to get a full picture of what will be going on.
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Finally, the same results should be obtained with the other cultures if they both have only one minor god that belong to a unique given main god (as Hera with Zeus). If there are none, the expectation will always be 0.75 common minor gods (like between Zeus and it's brothers). If there is more than one, then it will depend on how many exactly and the distribution of those among the main gods. We have to wait the info on the other cultures to be conclusive about that.
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À la guerre comme à la guerre