The world needs chemotherapy at least
https://en.wikipedia.org/wiki/Anthropocene
But joking aside, sometimes the world just needs you not to be a negative externality. Lower consumption fits the bill perfectly for this. If you've higher goals, there are just numerous vocations and causes to choose from.... And they don't need to be global. Being local and regionalized can make life fulfilling.
Sometimes people are lucky in that the last question comes to them. Think of the red cross volunteer who has an insatiable need to be always caring for people. They become a nurse, volunteer at veterans home, do EMT, etc. Other people are big abstract thinking problem solvers - maybe here is your professors, philosophers, etc. Others are hands-on must build stuff. Like engineers or those trying to build nuclear fusion or something like that. Then you've the organizers, guardians, leaders, just keeping the system running types - non-profit leaders, police/military force, politicians perhaps, government employees, etc. You get the gist.
I'd say since there are so many causes to choose from, its best to to find one where you can line up your personality. As opposed to say trying to rank them from least to most impactful.