I would really love to see Geroia, Balion, and Nordland/Jumne added as factions. Balion could be added to the island chain to the West and add the Republic of Geroia to the islands that are between the Empire and Aserai lands. With the map changing from what it was in Warband this would work. Geroia sounds like it is based off of Genoa or the Republic of Venice and is ruled by merchants which would be awesome for this type of expansion.
You could include a merchants guilds and this area is kind of like a Mediterranean sea area and their naval power is how they keep their independence from the larger factions. They would probably have the best ships and some early version of a hulk/carrack with lots of crossbows and swordsmen. They would be extremely rich and have some trade monopiles on certain goods. The faction could have a banking system and give out loans with interest or allow you to deposit gold and make interest. If you don't pay your loans things/accidents start to happen to your clan. They would have some troops but would use lots of mercenaries. Pavise crossbows could be their main troop type and maybe some light swordsmen. Their could possibly be an assassins guild that operates out of this factions island as well as they need a way to guarantee their loans.
Balion on the other hand would be like an early English type kingdom that is plagued with raids and invasions from Nordland. They would use longbows and the infantry would use billhooks.
Nordland/Jumne would bring the Nord faction to the game and they would be a raiding and trading faction. They would focus on Longboats and gather large naval invasions from time to time and target port cities.
Republic of Gergoia port