Family relations are also common. Since Zeus is Ares' father, their relationship as father - son can be a central theme. It could be about love, conflict, or the passing on of power and wisdom within the family unit. Another aspect is the theme of war and peace. Ares is the god of war and Zeus has to maintain order, so their interactions can be related to whether to start a war or keep the peace.
Another theme is the conflict between duty and desire. As gods, they have certain duties in the Greek pantheon, but their feelings for each other can create a conflict with those duties. For example, Zeus has to maintain order among the gods, and his relationship with Ares might be seen as going against that in some fanfictions.
One great Ares and Zeus fanfic is 'The Rivalry of Gods' which delves into their power struggles. It shows how their different personalities, with Ares being more war - hungry and Zeus more concerned with overall rule, clash in a series of events. The author does a wonderful job of creating a vivid world within the framework of Greek mythology.
One great Zeus and Ares lemon fanfiction is 'Divine Bonds' which explores their complex relationship in a very interesting way. It has well - written descriptions and a captivating plot.
To develop Ares, show his internal conflict. He's the god of war, but he could have moments of doubt about the destruction he causes. For example, after a particularly bloody battle, he might question if war is always the answer.