Hmmmm, if you were to make it into a full game... I'd probably say either units that are very unique and each have very special abilties, or a bunch more different units that you can unlock throughout the game (or both! Though lots more different units might be tough if you still to chess pieces). Otherwise just some more unique bosses, and then maybe if its a big game upgradable units throughout the campaign.
Again good job and good luck with the game!