I think you need to work on the fine line between realism and playability. It would be fun to play the part of a tribal leader who founds an empire and this empire goes off to conquer the world or something, but this would mean you would have to start very small and work your way up to controlling 1000s of provinces in one turn. Which would put a lot of people off.

An idea would be to play the game in stages. You start off as a bunch of wanderring nomads who have just settled down and you smack some heads together and become the chieftan of a group of villages and you win this regional round. A map coverring an area 50 miles squared or so.

The next round is a national round, sort of on the scale of Viking Invasion or shogun total war, where you play battles on a game screen modified from the last one, so units are larger and you play on a larger map. Perhaps making the units stupider so the computer can have 10000s of units on one screen. Once you've founded your kingdom you can join in the continental screen, something like the european campaign map and once you've won on this map you can go to the continental map, where you conquer the rest of the world.

Maybe you could even have the entire game running at once, so for instance you could be the celtic iceni tribe and the romans invade your little world who in the past 500 years have enterred the 3rd stage, whilst you are still in the 2nd stage.

There could even be glory goals or something, different ways of winning that speed up your transition from chieftan to warlord to emperor, but put you at a disadvantage when you finally shift up.

Maybe even, consider you are the romans in the as before said scenario, you as an emperor can take control over warlords in a region and influence their little country sized world instead of conquerring them. As the romans did bribe tribes against each other and made a profit.

These ideas are there to make the game in depth without increasing the complexity for the average ADD player.