A Kerbal Space Simulator sort of design simulator where you design and create giant robots to fight kaiju. Design elements would give bonuses to strength, durability, agility, you can choose weapon load outs ( specialising in ranged or melee, for example ), that sort of thing. You also design its AI, which you want to make advanced so as to be more combat effective, but no so advanced that it goes haywire and just becomes another rampaging giant thing that you need another robot to take care of.
You get a budget, depending on success, limiting your options and increasing the challenge. Having robots that can support themselves out in the field in terms of power and repair is also useful, as constantly patrolling robots can react to kaiju attacks faster and reassures the nations of the world, increasing popular support. However, more short-ranged robots that need to return to base to refuel and repair can be made much more powerful than their more self-sufficient counterparts, with more energy consuming big guns and parts to compensate. So it's a matter of priorities.
Plus, there'll be DLC where you control the aliens creating the kaijus, with a biological production and design interface, wherein you can send giant monsters on terror missions to destroy the human race. All good fun.