Frankly what this world needs more than anything is education. Ofcourse education about islam. So more people doing dahwah and stuff. Also more support groups for new converts to islam. I learn things the hard way so I know how much need for help in this dept.
Fewer people. Not as a result of any catastrophic disaster, of course, but naturally stabilized at no more than two billion. With that population, there would be adequate resources for everyone to not just survive but have good, fulfilling and happy lives as long as resource distribution was fair (a lot fairer than it is now), and without causing any significant environmental damage.