When I was studying chemical engineering in university, I tried to get faculty interested in engineering yogurt-producing bacteria (like L. bulgaricus) to produce Vitamin A (or an equivalent retinoid or carotenoid): a "golden yogurt" scheme like Vitamin A producing golden rice [1]. But, they weren't having any of it.
This could be useful for the 670,000 children who die [2] and 250,000 to 500,000 children who go blind from Vitamin A deficiency [unsourced].
The yogurt could be produced from dairy and plant stock (which is presumably easier for subsistence farmers to procure).
I had the good fortune of switching careers into software, so I don't have the wherewithal to do this myself.
If you happen to have time, I think you could produce pretty good results from engineering a Vitamin A producing strain.
[1] http://www.goldenrice.org/ [2] http://www.thelancet.com/journals/lancet/article/PIIS0140-67...
These guys are working on a more ambitious scheme to engineer bacteria to produce vitamin A in intestines: http://link.springer.com/article/10.1007/s12602-013-9133-3