Sounds like some undertaking. I'd check first to see if larger mods like Populum have any of the features you are looking for, I haven't really played Populum but it sounds like the kind of mod that would do such things. If you want to have a stab at any kind of modding yourself for CoE4 read the modding manual: http://www.illwinter.com/coe4/coe4modding.pdf

I don't think it would be that much of an undertaking. Theoretically all one would need to do is to copy the Independent ai and make factions for each group you would want to add. While I have not looked at the code I would assume that mob spawn is based on tile location/attributes, so all one would need to do is make it so that each faction spawns on the tiles they would be associated with. The factions of Hades, the inferno, the elemental planes, and the void already do this and are immersive enough in their "invasions."

I guess so, but you'd need to change the attributes for every tile and also to catalogue the indie spawns that occur from every tile, I don't know if that information is actually even known by anyone but maybe I just don't know. I guess some of it you could work out though.

I have a feeling all I would need to do is to copy the ai for Hades or the current independents and simply spread the spawn info out over those copies. I won't know until I try it though.

Well let me know how you get on. :)

I've thought about this for Populum and I'll give a general outline of what I planned on doing in the future. I think you'd need to separate out the independents faction into separate ones such as: bandits, nature, indep civilization. You'd need to clear the map completely and generate units/team-switch for each tile on initial map generation.

That was my intuition as well, if the existing independents faction is replaced with many smaller factions which all spawn on the Elysium overworld map, at the locations where their particular faction would be located, then the ai would more or less function as it does when Hades or Void beings enter Elysium. I wonder if it would be possible to change the new independent factions to change based upon world era to improve imersion: have the Dark ages factions be "bandits," "dwarves," "Merfolk" (for tritons and the like), "nature/beasts," "Great beasts" (To denote the truely powerful monsters from the rest of the world), "settlements/pioneers" (For all human settlements: towns, hamlets, costal hamlets, small towns, and farms), "Mages" (for mage towers and vampires--people who would have immense magical power and more or less be independent), "tribes/barbarians" (For the lion, deer, hyena, and bear tribes), and "Undead" (If one wanted the undead of Elysium to be different from those of hades); but have the Empire factions be "bandits," "imperial" (for imperial city, towns, hamlets, coastal hamlets, farms), "nature," "Merfolk," "Barbarians," "undead," and "Fuedal" (For independent castles and watch towers--to create the idea that their is some resistance to the empire [just something I would do if I made this]). I would need time to seriously look at the codeing for this game--most likely some time over the December holidays--but if it is possible to have different factions based on different starting dates I feel it would add greatly to the immersion, make the Dark ages feel like the world is mad and everyone is killing each other while during the age of the Empire the world feels united against outside threats (i.e. the player) or have a split between castles and farms/hamlets during the reign of the king to simulate the nobels ignoring the needs of their people.