one suggestion i made quite some time ago concerning "minifactions" (based on civs technically already present in the game but don't have their own factions, such as the Thracians) who could/would be present on certain maps and available either for destruction or alliance and allow the player that allies with them to have access to their units. this would extend to naval units as well, so, for example, on largely naval maps the Iberians would use their own transports but be able to ally with a non-player group (such as the Phoenicians) to gain access to some warships, the alternative being that, on a given map, they would ally with a player that DOES have warships and split the difference: for example, the Iberians ally with the Athenians against the Romans and Persians on a naval map, with the Athenians covering for their naval needs while the Iberians help defend the Athenians on land, particularly with their walls and towers since (iirc) the Iberians are the most defensive playable civ