I did a Lombard concept time ago, I’m not very good at balance and theorycraft but maybe it can help with giving them an historical identity…
I think their identity should revolve around infantry and siege, having a permanent army of sorts since that’s what an arimannus was, always ready to fight (they were in war with byzantines all the time in Italy) and a mention to their law system (rothari edict) separated from the one of Romans, specially the weregild Germanic law. Finally the gastald who was a landlord controlling Lombard duchies and a mention to their anarchy period (TCs built faster team bonus and military buildings easy to spam).
Bonuses:
Start with +150 food and +100 wood
Villagers cost 60 food but are created 10% in dark, 15% in feudal, 20% in castle and 25% faster in imperial age and they attack using bows (ranged attack)
Rams can garrison double units and get more bonus against buildings
Military buildings cost 50% less wood and are built faster
Unique units:
Arimanni (weak infantry that can be converted into villagers for 10 gold with a bonus attack Vs siege weapons and buildings, available in feudal age in town centers)
Gastald (fast infantry with bonus attack Vs cavalry, enemy monks convert slower when in his presence, available at the castle in castle age)
Unique techs:
Permanent army (available in castle age in the castle, arimanni take half population space)
Weregild (available at the castle in imperial age, for every villager of yours killed by an enemy you get 10 gold)
Team bonus: town center built at double speed
Missing techs:
Archery range: arbalester, parthian tactics, heavy cavalry archer, hand cannoneer, elephant archer
Barracks: eagle warrior
Stable: paladin, battle elephant, camel rider, steppe lancer, bloodlines
Dock: fast fire ship, heavy demolition ship, cannon galleon, shipwright (they get dromons)
Blacksmith: bracer, ring archer armour
Siege workshop: bombard cannon, armoured elephant
Castle:
University: heated shot, treadmill crane, arrowslits, keep, bombard tower
Monastery: ?
Economy: stone shaft mining, crop rotation
Architecture: central European in dark and feudal age to represent their migration and Mediterranean in castle and imperial age once settled in Italy, with the castle resembling rocca dei rettori in Benevento or the palace of Autari II
Wonder: San Salvatore basilica in Spoleto
Language: Lombard, maybe not enough info since it’s dead like gothic
Campaign: probably Alboin