I think making the AI better at claiming victory when it is close to it's chosen victory type should be given priority. Playing on Immortal I sometimes see the AI well ahead in Science and it builds 1 or 2 Spaceship parts but then it just stalls completely. The AI should win a Science victory if it is significantly ahead in Science. I've also never seen the AI win a Culture victory, i'd like to see that change in GS. If the AI has a bucket load of culture (think Pericles) the only challenge that brings ATM is if you're going for a culture victory, delaying your culture victory enough for another Civ to win a Science victory.