More than visual differentiation (wich may help, but I find secondary) is the stats differentiation.
It has been said some posts before, and I will add more suggestions:
- Unique units. Each faction should have at least 2 or 3 unique units with an specific use. Like jugs. Like hench. Like slaves. I miss monks that can heal, magicians that can use one or 2 spells (attack or defence), druids that can entangle or daze...I think it can be easy introduced by adding traits. Many of the special units are deep in the techs, so it could be good a early specific unit (weak) and a powerfull unit for mid/end game. With a magician unit (monk, warlock, summoner, druid, necromancer, vampires with charming....) we can have now 3 specific units for each faction.
And that, only by using "humanoid models". I think a faction could train ignis or ophidians, skath, spiders, ... almost any of the monsters... But don't attach it to camps.
- Unique traits. One or 2 specific traits for each faction, that will really make difference in standar troops. Many of the traits are already created, just give some of them to a specific raze. Other traits could be like some of the skills of heroes: heal (for the monks), swipe, double attack, ... That can really make different units with different uses.
- Mounts. Using spiders, bears, or even skaths could add more variety. How to implement new mount techs? Don't do. Change current to: Basic mount (half bonuses) and Advanced Mount (full bonuses). I find that splitting wargs from horses is a mistake. With my suggestion, any mount can be added without new tech requiered.
- Specific technologies and specific buildings. Only by changing the order of the tech trees for each raze, would make playing each one more different. Like making bows starting sooner in tarth. Change the % bonuses for the buildings in each raze: more magic for the magic razes, more production for the ironeers, tune the research, the food, the unrest, the money... that each building provides for each faction... You can even customice the metal and crystal (instead of 0.5, give some 0.3 and others 0.6). Light changes like that makes light changes in each gameplay...but become big changes when you see all them together. And all this, without creating new techs. Of course, creating new specific techs would give still more difference. Balance issues? Well, we are here to test them... But at this moment, balance is the least important of the issues.
- Heroes. Yes, they need some differentiation. I find many of them with 2 or 3 schools of magic. Give them traits form different paths (like administrator, researcher, or others like stun), to expand the difference. A mage with double strike? Well, it may sound strange, but why not? Why all mages are evokers? Mix them, give them personality, and then let the player decide what future wants for them. Don't close the traits from other paths, because if not, all mages are the same, all assassin etc.
- Weapons. All swords with counterattack? No please. Give each weapon a unique feature. We can play with stats like accuracy, dodge, critiks, % armour pen, and initiative, to make more different each weapon. More wapons exclusive for each faction (without having to select assasin tools, or athican, or whatever. Just because their faction uses them).
So, except for the new mounts and the new traits (some become new units , all I have suggested is already there, just is needing a tuning.