Originally Posted by Rosacrux redux
Mine would be an RPG-TBS-RTS (a-la Rome the latter) hybrid, with some adventure elements. A game of grand strategy but also of petty politics, humble starts and stuff. Say, let's name it... Alea Jacta Est.
You start in a historical setting of choice (I would actually choose the ancient Mediteranean world to be represented in the game, very in-depth and accurately) and pick up a character. Not a historical one. You may pick a farmer in 50 AD Rome, or a peasant in 4th century BC Macedonia, or a citizen in 5th BC Athens, or an omoios in 6th BC Sparta or whatnot. Then you try to bring your character to a position of power. You can use military skill (at first, you'll be fighting in the lines with the other soldiers, than as you advance in rank you will be assigned various commands, until you manage to have your own army and be in a position to claim higher power) or political/diplomatical skills (endless sessions at the Agora, ostracising, backstabbing at the Senate, scheming in the court of some hellenistic king, panem et circensis, winner at the olympics... you know the stuff). Eventually you can come to a point to be absolute ruler or be the strong man of the republic/democracy... and here is when the real fun starts.
A great asset would be to actually build up power for your family. Say, in the first generation you gain military fame and some money and influence. In the second a seat in the senate. In the third, you become a famed politican and Dictator... that kind of stuff.
It's a game of uber-epic proportions, I might say... would be the absolute game. With a battle engine similar to RTW (or MTW 2, even better) but you can be able to guide your individual character... and lots of other bells and whistels... while the political/diplomatic model would be... well, don't think there's any game featuring such an engine...