Make Mars herbivorous
Soon we will colonize Mars and start terraforming it. We will seed it with vegetation and bring non-human animals to create a new biosphere. We will face the decision of which kinds of animals to take to Mars. Of course this will depend on which species are suitable for the environment, but we still will have many options for the sort of nature we want on Mars. We could replicate the long-standing mixture, or "food chain", we find on Earth, or we could take solely non-violent, herbivorous animals. Why create violent nature when we don't have to? This would be an amazing opportunity to create a vegan world characterized by plant-eating.