Something that has come up a lot lately is the current alliance system. Right now in order to ally with anyone you need a late game tech and if you ally with all the remaining players you win. This has several problems. Firstly the lack of early and midgame alliances prevents a lot of strategies and really limits your interaction with other factions for significant parts of the game. Secondly it causes a lot of weird situations when it comes to alliance victories. For instance if you go on a rampage and kill 5 players but have one tiny ally you overlooked you get an alliance victory, because apparently you are a master diplomat. Secondly, you can't unally people to get conquest victories, you have to wait for the treaty to expire. 

I would suggest splitting the alliances tech into 2. A earlier tech that unlocks alliances and the later one that unlocks the victory condition. This would allow players to fully interact with AI's much earlier but still prevent games from ending too soon. Secondly I would suggest changing the condition for alliance victories slightly. Make them require that the winner have less then half the total faction points. This would prevent players that are clearly dominating the entire map and could easily crush every other player from getting a "Diplomatic" victory. If you could just demand the surrender of every other player it should give you a Conquest victory instead. Also their should be an improvement like a wonder called the Sovereign's Council, with a high influence cost, that you have to build before you can get a diplomatic victory. This way alliance victory becomes more then just a side category of Conquest and would require it's own strategies. Having more differentiated victory conditions would really help with replayability.