Egyptian 4th Major God
I think it’s the next after the Norse and the Greek, then the Atlanteans.
Two options so I voted for the Celts and the Slavs
Celts
Historically in connection with other civilization Greek, Norse (Germanic people). But I don’t want to see arthurian legend because it’s too tied with the middle age, I don’t want to see units who look like late middle age western european knight with Plate armour in Age of Mythology.
Seeing historical figure like Vercingetoryx why not but I think there is many mythological hero available. Fictional figures who look like Asterix and Obelix (menhir thrower unit) as campaign hero why not.
Hindu
They have all to be a good addition but very sensitive theme because is actual religion worshiped by more than a billion of people.
Mesopotamian
The architectural set and mythology is very well documented, can be very good in Age of Mythology. Sargon can make a comeback since Return of Rome.
North american pantheon
Can be good specially with horse unit (great plain indians from popular culture)
Polynesian
A good thing is that we can take all the various people to make an umbrella civilization : from the Maori, Taiaha warrior, Mere warrior, moa rider, moa caravan, Pouakai (mythological bird inspired by the real Haast eagle), Taniwha, Patupaiarehe, Ponaturi.
From Kiribati their warriors wearing basket armor and shark teeth weapon
Moaï from Easter Island. Hawaiian like warrior.
Slavs
Can be good, just see the illustrations of the slavics gods (specially the one of Svarog) by Andreï Chichkine
Tengrism
Some people mention this mythology. Can be very good, imagine a Genghis Khan campaign but with mythological units, this will be awesome ! This is the type of campaign where you can fight all the other pantheons.
With the possibility to unpack and move buildings.
The unit are obvious : Keshik, mangudai, horse archer with horse wearing ibex horn like the Scythian archer of Chronicles.