I guess i should clarify the NPC options I was also imagining Tyr/random character in the background, like you first start in a lab, they are in a test chamber and you have to grow them out of it while surviving waves. then they would jsut follow you around as you do different things to make them grow bigger. even tualy becoming kaijus and breaking out onto the streets then make a rampage in the background whiel you continue growing them bigger
