How about turks/bohemians+khmer+celts?
The bohemian player is the anti siege and defense specialist using their amazing bombards to destroy enemy siege and placing bombard towers around your side for defense, they also have good monks for conversions if needed.
The khmer player gives your team the opportunity to attack in castle age in case you match up vs goths or something, they also buff the celt scorpions and have ballista elephants which are pretty much unbeatable.
The celt player has the strongest siege in the game, and with the khmer bonus they get super scorpions, they also have infantry to defend the siege