In that case, I second the ability to allow air units to sink ships.Also bulking up air vs. navy aspect.
Also, bring in more units that don't require a resource - or make iron not be such a monopoly on military production for units. As it stands, Iron is all you need until oil, then you need both, or oil+aluminum. Not having iron, means you don't get to build most units in the game. Crossbows should be without iron IMO.
If you want iron, go grab it.