Don’t make the new civilizations too different! It takes a lot for a new or a returning player to learn multiple different economies and unit types that differ a huge amount from the other ones.
Having a standard way of playing the basics of each civilization is good, and I understand that to a large extent this is already true of AoM. The biggest differences being that norse villagers are unablento build and that atlanteans have very different villagers from the other civs. I think this should be changed. Norse villagers should also be able to build and atlanteans should have regular villagers imo. I will give you an example of why for each:
If you play a 3v3+ team game on Anatolia and you get the middle position as Atlantean, you are at a big disadvantage because you can’t go build a dock and gather resources at the start. While other civs would send 1 villager and setting up the economy at home, atlantean effectively sends 3 to the dock or waits for the next villager to train.
As norse if you are attacking and someone earthquakes your home base you can’t rebuild it with villagers like every other civ can, making some god powers a lot more effective on Norse. Also norse builders effectively cost more population, which is unfair in the late game imo. Another problem with this is that you can’t really choose certain army compositions as norse because of this. You can’t go pure cavalry, myth units and siege, because then you can’t build anything.
Some of the human troops in AoM are pretty bad too. Peltasts are unusable, slingers and turma are horrible after 10 minutes in to the games katapeltes are pure counter cavalry, but are too slow to deal with them and just die to anything else they come across.
Myth units you can go crazy with imo, since they are unique and should be given unique features, abilities and strenghts.
A few of the myth units are pretty bad, and should be worked on. For example the minotaur and the cyclops can’t do anything if you move right before their special gets off. Son of Osiris has the same problem at long range. The Troll, the Wadjet and the Phoenix are underpowered and the Scarab needs Eclipse to be useful. The one hit knockout myth units should be equally fast (the Mummy, the Lampades, the Medusa).
Also here is a video that you might be interested in:
It’s a comparison between AoE2 and AoE3. It’s not directly related to AoM, but there are some things that you can get ideas from when working on AoM Retold.