Future expansions.

They could at least add a ninja unit.
So many things could be added to it – it would look colorful – throwing shurikens, disappearing in clouds of smoke (teleportation like vampires), using a katana, chakrams, a sword, chopping enemies to pieces, dealing unavoidable damage, poisoning with venom, dealing multi-damage like a hydra.
The closest analogue is the thief from Heroes IV, but he is very weak there.