I think another solution to balance thralls is to let players choose a class when building its character. Each class would define differents gameplay particularity:
Warrior: it gives the abilities to the player to have great attributes in fight: Exclusive Heavy Armors, Exclusive Heavy Weapons, Exclusive Cure Resources, etc.
Pokemon Trainer (sorry, couldn’t thought another name): it gives the player the abilities to tame really strong warrior thralls. This Thrall should be loyal only to the player who gives him special training: Abilitty to build barracks to give thralls specific training, ability to create special foods that improve thrall atributes, Ability to level up thralls with no need to take them in real battles, etc.
Worker: it gives the player the abilities to improve farming: Exclusive tools to farm faster, etc.
Architect / Engineer: it gives the player the abilities to create stronger buildings: exclusive materials, exclusive reinforcements, etc.
So the player should choose the class according to its gamplay. And it would give more reasons to build a clan, once each player should offer unique contributions to the clan. And Roleplay would be even more nice.