Nice thing to theorycrafting, but personally I don't see it happen. For fun I would see it as a nice oppurtunity to discuss though.
So let's say every Civilization would receive special new resource (favor), building, 2-3 units (at least 1 flying one from Age4) and titan. What it could be so?
GREEKS would train those myth units in Temple and Villagers could gain an ability to pray there to gain Favor. Pegas riders could be their flying unit and Atlas could be their Titan.
EGYPTIANS would train myth units in their 3 temples (every temple different unit). Also Priests could summon Obelisks that generate Favor if there is no another Obelisk nearby. Their Flying unit is Sphinx and Titan like this one from AoM.
PERSIANS would summon myth units by Maguses and gain Favor by special War Academy Toggle tech which generate Favor per second in cost of resources (more resources means more Favor per second). I'm not much into their religion culture, but surely something cool could be found.
CELTS would get Favor as another huntable unit sacrifices and their Myth units could be trained in Sacred Grove. Not sure about the units at this point yet.
BABYLONIANS could get Favor by Garden special tech, which will allow Gardens to generate Favor per second. Not sure about the units at this point yet.
NORSE gain Favor from killing units (based on their housing capacity) after researching a technology in Age 3. Their Titan is Giant.
ROMANS could get Favor by Priests by Relics which could be found on the map and delieved by Roman Priests to the Templum.
INDIANS could be able to choose the way how they get their Favor, but it would be something related to Sacred cows, herdable units and Shrines.
Mythic units could started to be trained in Age 3. Mythic units costs always favor, but never only favor. Favor can't be traded or tributed. All players starts with 0 flavor. Titans are trained in Wonders and each player may have only 1 Titan at the time. They are very powerful, expensive and have long training time.