One limitation that games like Civ suffer from is that diplomacy is ultimately pretty shallow because there can only be one winner, so even when you're building alliances or trading relationships it is generally to gain some temporary benefit until you are in a position to defeat your partner later on (whether militarily, scientifically, etc).

What I would love to see is a multiplayer game like Civ but where each player has independent win conditions (so that a game could have multiple winners, or no winners). The condition could even just be to attain a certain level of happiness or wealth. And if you achieve that then you win even if other nations are bigger or stronger, and conversely if you don't achieve it you lose even if you are the last nation standing. So decisions to go to war, or focus on technological development, or build alliances or trading relationships, etc, are driven by the wants and needs of your own people and not just a need to dominate others.