1) Please make the AI always value constructing metal-producing improvements and upgrades whenever possible. This is crucial to making sure the AI can build stronger troops.
2) For the middle game, also make sure they improve crystal-producing improvements for the same reason.
I am tired of facing leather-wearing spearmen on the highest difficulties. These things are all I face because the AI gets insane research bonuses, gets the highest level armor/weapons unlocked, then can't build them for 150 turns because they lack the resources to use the auto-upgrading unit designs.
Other than that, I strongly agree with the previous posters that Strategic spellcasting is what needs to be improved. Tremor and Freeze are excellent opportunities for improvement, tidal wave also. These things should happen automatically as soon as I step inside AI territory. It will give the AI a huge boost, since it means it will get 2-3 more turns to gather up the defense. I haven't had these spells used on me since beta 3.
I would also love to see the AI use the high-end strategic destruction spells such as Shadow World and Curgen's Inferno more reliably. I have several times seen AI sovereigns running around with these spells unlocked, but I have never seen them used. I respect that they are very costly to use, and obviously it should not be a simple case of "if I have the spell, always save up mana to use it". But they are sooo devastating in power that they really should be put to use if you can.
PS. Please do add notifications for when the AI targets my city or my units with any spell. Unless I am actually looking at the place in 3d mode, I will miss the effect play and that is all the notification that currently exists.