The game right now has selected buffs that work on all allied units such as the Mongol Kurultai, Khan, Jeanne D’ Arc and other units. Movement, attack speed, and damage/armor should not be selected buffs with different civs. It would enhance team games if the English tower would also buff allied units, same with the Mongol tower buffing movement speed, or the Ottoman Mehter. It doesn’t break game balance at all for 1vs1 and team games will only be better for it.
All they need to do for balance is not have the allied buffs stackable and only the best movement, attack speed, or damage/armor buff apply.