Or how about a law that says if you've rented more than twice the value of your home, it becomes your property. We could slowly get rid of the robber barons. Being secure in the knowledge that you won't be kicked out of your home just so someone else can make a few dollars would make most people very happy indeed, I know it would take a load off my shoulders.
First of all, because I can feel that we as a community accept science as a guide for our actions when science really calls for it. Second, because I feel that between all the conflict in the world, we can come together when it counts.
Other people who are miserable and net unhappy you could massively increase their happiness by making them have a net positive emotional valence. So if you want to increase the gross joy of the world, focus exclusively on those most unhappy. This has the caveat to help those who can be helped and made to feel happy of course, and most long term impacts. Fund wells and clean water, give job opportunities, improve social care, education, surgery for people in developing countries with terrible quality of life (e.g. women in Africa with horrible labour related tears and constant incontinence- that’s awful and relatively easily treated).