I'll be a little bit of a contrarian here, if you don't mind.
First, I think it's great that you and your investor are trying to think of ways to make the world a better place by encouraging humanitarianism in others. That's a really tough challenge, as others have pointed out, but I think it's even harder than anyone is giving it credit, and that's where I start to get worried.
I think it comes down to measures of effectiveness. Let's pretend that you land on an idea, build a game around it, launch it, and deploy it into the wild. One of the objectives of the game is to make the world a better place. How do you know whether that effort was successful or not? How would you measure that to see if you had an effect or not? I think when you launch your game into the wild, it's also going to be based on a lot of assumptions, many of which may turn out to be wrong, so I would expect to see the launch act as just the first iteration at the problem rather than the final solution to it, which means there will be lots of version updates. And, the version updates will need to depend on knowing whether one version was effective or not in changing the world in some effective and meaningful way -- otherwise, you're just shooting in the dark, hoping for the best.
Forget about games for a moment and take a step back and think about "making the world a better place" as an agent of change. Pretend that your investor has given you a hefty sum of money and said, "make the world better somehow". What could you do to get the most bang for your buck? How could you make each dollar have the most possible impact? I think, to clarify, when we say "saving the world", we're talking less about the dirt, rocks, water and air which composes the world, and more about saving the people and other inhabitants of the world (life). In the most broad sense of "saving the world", what we're really trying to do is make living more pleasant for the life currently on earth, right? That could be something as bold as creating a habitat refuge for endangered species facing extinction, to something as simple as encouraging people to smile and say nice things to strangers.
There's already a huge "game" like this happening in china as well, where everyone in the country is a player in the game whether they want to be or not. They have massive surveillance, cameras doing facial recognition, and have begun giving people and their life profiles "digital points" for different social behaviors. This is treated like a credit score on steroids, but it's for social credit. If you have low points, you get low priority treatment, such as being put on the back of the train, denied loans, job interviews cancelled, hassled by law enforcement, etc. Maybe if a camera catches you littering, you'll lose points. If you owe people money, you lose points. If you're late to work, you lose points, etc. It could become the perfect case of a road to hell paved with good intentions, but ends up backfiring because rather than making living more pleasant for everyone, they end up making it worse? I would worry about less than benevolent uses.
Anyways, you're probably looking for game design advice, so here's my advice:
1) Aim for really, really low goals that are achievable. Changing minds or attitudes for the better might be enough.
2) Avoid complexity like the plague
3) Be scientific about how you approach your assumptions (hypothesis -> collect data -> test -> review -> repeat)