personal finance management could be the next solo project. its one of those areas that is honestly, genuinely useful. money is a huge motivator for many people and better management of the money you've got is the same as making more money. so in effect the site gives away money for free. its also an area that people are chronically undereducated about.
the current gig should end in august and then i'll have free time for a few months to do such a thing.
most important things clean water/air/food affordable housing a fair legal system