I have a recommendation; make a mod which is set in the British Isles (UK and Ireland) in the Iron Age and with about forty factions. These factions would be the Ancient British tribes (e.g. Iceni, Dumnonii etc) battling for dominance of the islands. Perhaps the Roman Empire could be an extremely powerful "emergent faction" arriving in boats in 50BC led by Julius Caesar, landing in Kent.
Thye campaign map would use the map of the "traditional counties" of England, Ireland, Scotland and Wales as a basis and then sub-divide those units into three or four pieces, equating the province and region dynamic. As for the appropriate Iron Age names of the provinces... I would retrofit using the Welsh and Gaelic languages as a guide - ask me if you need guidance. The factions could differ from each other by being parts of about four different "cultures". The Britons of the south-east (Atrebates, Catuvelauni, Cantii, Regnii, Dubonni) would be "Belgic" and use money, share religious traits (e.g. worship Beli) and live in "Oppidum" (towns). Some other ancient British tribes would have a "Brythonic" culture and have different gods, where appropriate live in hillforts (e.g. Durotriges, Dumnoni, Votadini, Ordovices). Other factions would have "Ivornian" culture, different gods, different appearance (e.g. Irish factions, Iceni, Brigantes, Deceangli). A final group might be "Caledonian" culture and include the ones living in northern Scotland.
I think this would have the advantage of:
a) being smaller and more manageable
b) being a time period and locum which is popular
c) offer diversity of units and cultures