I'd pick several issues---low-income folks in need of legal services, decline in formation of new small businesses, need to rethink and retool education for a new 'Conceptual Age' in developed countries, et cetera--and solve them. I'd pick solvable problems, and then find solution(s). 

I think the best return on investment a billionaire can get from their philanthropy for people to almost forget that the problem solved was ever something people struggled with. Examples could diseases that have been taken care of, at least for the most part.