A few ideas:
Ways to make having an excess of resources useful - it can be somewhat annoying if you have 6 iron but nobody to trade it to, and lessens the incentive to seek out more resources once one unit has been found. A couple of ways this could work:
- allowing buildings that consume a unit of resource. If more buildings are constructed than there are units of a resource available, then there is some sort of penalty eg. if you construct 6 nuclear reactors but only have 5 uranium, then they only give 5/6 of the normal production bonus. You'll have to keep finding more uranium if you want to have lots of nuclear reactors and get the full benefit.
- allowing dynamic expressions for unit shield cost. Eg an iron-using unit might have cost along the lines of 20 + 20/x where x is the amount of iron available. The more iron a player has, the faster it will train, but with diminishing returns.
Some of the combat aspects from other civ games. Particularly having wounded units fighting more weakly; this could be done either by making each hit do less damage (ala Civ 4), or scaling attack and defense appropriately. Best made optional in the form of flags (i.e. "Inflicts less damage when wounded" and "Less attack/defense when wounded").
The "Firepower" stat from Civ 2.
Combat that doesn't necessarily result in the death of either unit, via an "Attacks" stat. When attacking, the combat will go for a number of rounds equal to the unit's "Attacks" value, and then stop. Skirmisher units might have a low attacks value, with berserker type units much higher.
Option of unit per tile cap.
Ability to alter the city radius and minimum distance between cities.