My suggestion is to not listen to the myriad suggestions to make the gameplay harder in different ways. I'm not saying that you should ignore people's opinions, just that I would like disagree with a common bit of input. I see a lot of things like "Jack the unit costs up super high so that it's harder to make big armies! Muh realism!" I really dislike things like that. A game's challenge should come from things like proficient AI opponents, not "artificial sweetener". You guys have done a great job at making a more entertaining AI to fight, so keep that up!