This is quite biased, consider I love making Civ designs.
but you can never have to many Civs. in my opinion.
Civ = Content.
You can never have enough Content.
Now when it comes to balancing, yes It can become problematic, but nothing to impossible.
For people who have it in their mind that it is impossible to balance infinite amount of Civās.
You are correct.
But perfect balance in itself is impossible to achieve, unless you sterilize everything leaving you with only a single option. Only then is it perfectly balance.
Will things become more or less balance?
That all depends on your perspective. With great assymetry, comes great challange, not just in balance but also integration. But again. nothing that isnāt any more challening than the usual when doing a project.
But AoE has a solid core fundation. As long as that is kept intact, any ideas and mechanics built upon that, should be able to be towed in line, and over time balanced out.
Yes, there will be Civās stronger vs some Civās naturally. we already have this in game.
But then it becomes a matter of Perspective, and how you interpret Game Balance.
In my opinion, what is most important for game balance is Enjoyment.
In the end, if I get great enjoyment out of it, I couldnāt care less if French were its hardcounter and it only has a 30% winrate against french. If the Civ is really fun to play, it outweighs its difficulties against the French.
Over time, they will find out things that make them so weak against this specific Civ, they will add things to help adjust it a bit.
So my Conclution.
You can never have to many Civs.
It what makes diversity, and Diversity is fun, its one of my top favourite things about games such as AoE2, Civilization, even Total War (before warhammer bleh)