Here are somethings I would love to see added:
1) A Attribute for city improvement game modifiers, called AdjustAttackingUnits. Currently the game only supports adjustments to defending units, AdjustDefendingUnit, but what about improvements that have an effect on attackers. Like a boiling oil improvement that make attackers take damage based on their unit size ( a lot like the Artemis System Net in MOO2) or even some kind of tactical mana regen debuff.
2) A tag for city improvements which is the makes two buildings mutually exclusive. Currently, you can make one building require another but you can't directly say "If you build a merchant, you can't build a study." While the mod community has figured out some work arounds, it would be a nice addition.
3) An Attribute for items to add strategic spells as abilities, much like UnlockCombatAbility. I would love to be able to do something like UnlockStrategicAbility so I could create various strategic units based upon their equipment.
4) Expand access to current ability scores. I would love to be able to access a unit's current moves, in much the same way I can access their current health via curHealth.
5) Add some method of letting tech give items, units, or improvements directly. I think it would be cool if I could research a technology which not only gave access to these things, but actually let you bestow them directly based on certain criteria.
6) A duplicate emitter button into the Particle cauldron. Several times I have wanted to make a near identical emitter as a previous emitter, but had to go through the long process of adding and updating key frames to do this. It would be nice to be able to just press a button to duplicate the currently selected emitter.
7) An increase in the upper bound of the move y slider for the builder's forge. There are actually a couple of building models which are so tall that an item placed on the ground would need to move the slider twice to get to the top.
8) A method for having prop units move around in a set path on tiles. I love the prop units for tiles, but I would love for them to be able to move around the tile a bit. I think this would enhance our ability to make more lifelike city improvements.