Though i agree with you that Food is actually getting wasted in IA, i would disagree with using it for walls upgrade. I would rather have an exchange market to exchange food for Gold or Oil. It can work in the similar manner as it used to work in the Age of Empires and Rise of Nations. More food you sell to buy Gold/Oil higher will the exchange rate become and vice versa. Other good way can be introduce using multiple items for upgrades like Gold + Oil for General Upgrades, or Food + Oil etc.