There are a lot of units you could potentially add into a game like this. This could be taken way past fan service and go to the point that you are adding in awesome, new units. Maybe you add in pegasus and wyverns so that you can make your own large-scale, live action Fire Emblem battles. Maybe you add in cars with their own control schemes for accelerating and braking. And these could even be made such that normal infantry could "mount" them.
You could even add in mythological gods, such as Zeus, Kratos (I dare you to tell him he doesn’t count) or Sun Wukong, the Chinese monkey god.
There could also be some characters that have unique options for navigating the world. There could be characters that jump really high or fly. The system seen in Prototype and the Saints Row games, where you run straight up walls without the need for premade handholds, could be great fun. And Grow Home/Grow Up’s system, where the character grapples directly onto the drastically varying level geometry, could also work.
Imagine a hoard of zombies climbing over every nook and cranny of the map slowly working their way toward you like a tide of cockroaches.