This mod would probably start like an RPG/Adventure game, the cost to build cities would be very high. So the sovereign will roam the land, and after many adventures, gain sufficient funds to create his first city, and then, build an empire.

 Is there any interest in this kind of gameplay ?

I think this has a lot of promise. A little like Mount and Blade, honestly.

Sounds good.

you should take a look at the Make-An-Empire thread

Some more details of what I would like to achieve in this mod:

- High Sovereign/map interactions: the sovereign will have many skills (bounty hunting, lumbering, mining, building, forest knowledge, animal taming, hunting, herbalism, and so on): These skills will improve what he can do, and harvest, in the world, what he can do with NPC's, and so on. So the idea is that at the very beginning, the sovereign interactions with the world will be very limited or quite ineffective, but as he grows in knowledge, he will be able to do more and more things, and much more effectively (so, with no herbalism, he wouldn't recognize this "batch of herbes", and as an herbalist master, he would be able to harvest quite a good amount of it, either for basic healing, mana gaining, temporary buffs, or just sell them to a merchant caravan. With the proper "alchemy" skill, these herbs could be turned into more potent but expensive to create potions.

- Object crafting, with the skills. Hopefully, I will manage a UI where the Sovereign will be able to craft objects. The main idea here, beside crafting, is that the sovereign will not be able to precisely know the value of the object, he will only choose the overall quality of it, but the better skills the sovereign will have, the better the object will be. For example, lets say the sovereign is an apprentice weapon smith: he could choose to create only "basic" sword, with some chance of success. This sword would grant a bonus between -2 (a malus, as a matter of fact) to +3. This sword would need some components (5 materials, for example), and require 2 turns to create. Then, as a master weapon smith, he would be able to try to craft "legendary" swords, with bonus ranging from, say, +10 to +15, requiring 100 materials, costing 1000 gold, and 10 turns to craft. This should offer a huge replayability to crafting.

- An "exhaustion" value, so the sovereign will need to rest sometimes. Not in my priorities though.

- Random techs trees: at the beginning of the game, hopefully, all techs within the various domains will be accessible, (no empire/kingdom specific techs paths), but each time a tech will be discovered, another one will be randomly discarded, such making all civs, in the end, totally different, and offer the game a huge replayability. This implies to redesign the techs for autonomy and balance. This was the main subject of my civ4 "age of civilisation" mod.

- First part of the game heavy on RPG stuff, and slow strategy. The player shouldn't be able to build his first city before many rounds, and then, he won't be able to build many cities. From a rag-tag adventurer, you will hopefully end the game leading an empire. Think Mount and Blade, but with the depth of Elmental.

A) How will these skills translate into the later game? For example, what will "herbalism" grant you when you have 10 huge cities?

Well, I think the "pleasure" of this mod will be more Sovereign RPG centered than "Empire building" centered. Anyways, end game, being a powerfull herbalist, you, as a player will be able to harvest the more powerful herbs of the world. It might not be very relevant, of course, and an empire shatering skill, but those are my first thought. It really needs to be tuned to the scope that E:WoM offers. But what if these herbs are required to cast some potent nature spells ?

B ) Could you not hire followers to do some tasks for you?

That would be cool, why not, but it might require some advanced AI programming. Anyways, recruiting a specialist NPC in your party might give you his special skill. Anyways, at first, I will more focus on the level of interactions of the sovereign himself, with the world. Any action (even recruiting), will be tied to some skill, and improving these skills will either allow you new actions, or improve the basic actions you can make.

C) What about diplomacy in the RPG stage? Couldn't you just run after and murder the other sovereigns?

Good point, that will require some balance. And why not conquer an empire with the might of your sword (conan like?). I think the pace of the game will need to be pretty slow, and the diplomacy refined, so that the RPG sovereign does not end after a few turns, chased by whole armies, that would spoil the fun.

Anyways, the "random tech tree" looks interesting. I'll be interested to see how it turns out.

Statisticaly, no strategy game will hardly ever be the same with this system, and you will end with civilisations as diverse as the vikings or the mayans, or the native americans, or the polynesians, all very specific...

I'm highly interested in this idea for a mod.