Proposal for city-states:  - citizen soldiers have better gathering and combat stats (not sure which ones to improve), but are more expensive. City-states had limited manpower compared to larger polities, but they had higher civic spirit. - population cap is reduced by 10%. Similar thinking as above.  - additional civic centers cost 50% higher. City-states did not expand like empires. All proposed city-states already have the theater to expand original area. Note: all of this is a nightmare to balance but worth a try in a mod 

I am certainly not an expert by any means, but I believe, the Germans are potentially a good candidate (if we refer to their migration). Not so sure about Gauls or Brittons though.

In general yes, but Iberians have stronger towers than other civs in EA. They can be the exception though. Exceptions make the game fun!  

Which civs would fall into this category?

Well, they may be initially nomads. Also hunting could have a little bonus (they might therefore have two or three scout cavalry at the beginning instead of one) and farming could be available only later after technologies have been researched? Not sure this woudl be changing much in the long run but it could nudge players towards a little different initial strategy.