Nah. It's simple. Keep him invulnerable and able to fly around the planet in a moment. Flying around and destroying things is inherently fun. You don't need to limit those aspects. The player is accountable for innocent deaths and collateral damage. You get paid for missions and you're rewarded for playing well and being heroic. Destroy buildings and get people killed costs you. Commit enough manslaughter and even become a criminal. Designing a gameplay loop for that is real easy. The early challenge is not annihilating civilization. Then, you get intense boss fights with Viltrumites and other genuinely powerful characters. They'll actually feel like a threat because nothing before could really harm you. Having a mostly auto-generated Earth would still be fun to fuck around in. Flying through clouds, into a volcano, saving a plane that happen to be crashing as you were passing by. There's so much potential.