This topic is of great interest to me. I like the idea of role-playing a nation/leader. Establishing some large territories with access to vital resources is great, but then development of my society as opposed to an unending territory grab is something that I prefer for my late game.
A couple of concepts to consider. In real life, if we model the development of the world from -4000 BCE to 2000 CE, active colonization occurred from -4000 BCE until around 1600 CE. That's 93% of the "game." In the Civilization series, a settler becomes a pretty much obsolete unit by about the 10-20% point. After that, you might build one or two to take a desert spot that just turned up uranium or something, but even that is unlikely with the way borders work in the game.
So I'd like to see colonization remain a factor through the majority of the game.
In real life, itsy-bitsy teeny nations can develop powerfully. In real life, Japan and Germany have stronger economies than Russia. If the real world worked according to Civ rules, the strongest nations would be the United States, Russia, China, India, and Canada. Germany would be lucky to be able to compete with Brazil in economic strength.
I'd like a strong sense of balance of power and nationalistic identity. Wars should be able to end like World War 1/2, where losing nations maintain territory, or some portion of their territory, but agree to consessions like a change of leadership, economic agreements, etc. The more complex, the better. The world learned that it was better for everyone to end WW2 with global reconstruction led by American corporate interests, rather than the resolution of WW1 which involves strict penal reparations. For the US, or even Britain or France, to hold and occupy Germany/Italy/Japan after the conclusion of WW2 would have been a mess, to say the least.
I realize that a lot of what I'm raising is more "modern" in nature than the scope of Elemental. They're just ideas to consider. Looking at the medieval scope, though, permanent steamrolling was still never feasible. Napoleon probably came closer to it than most others, but the borders of France were not permanently extended. I would like enough factors in place so that border wars, resulting in the acquisition of 1,000+ square miles of territory (a fraction of the size of New York state for Americans), are feasible. Larger acquisitions would need to be backed by positive sentiment in the conqueree's population or an extreme technology/economy difference with minimal balance of power concerns (i.e., the European conquest of North/South America). Holding such grabs should be exceptionally difficult though, as in real life.