I would like to see a conglomeration of existing transgenic technologies into a triad of species (2 marine, 1 terrestrial).
One would ideally apply the technology currently feasible for rapidly-growing, super-massive fish. This we already can do, manipulating salmon (or any species we select for taste, among other desirable qualities) to over express natural growth hormones and growth hormone receptors, resulting in a livestock that matures as quickly as a non-transgenic specimen, but also being capable of reaching a mass (or muscle mass) 5 or 6 times what we observe in nature. This is by far the easiest step in building the triad, but is still insufficient to meet the nutritional needs of a human.
The second member of the triad would be a terrestrial "emergency" species, one that would have be compatible with climates around the globe in the event that our primary marine stocks ever become unavailable. They would be maintained in something resembling a nation's strategic reserves of resources, not utilized except in dire emergencies or following a cataclysmic, malicious attack by a terrorist organization or a rogue state. This contingency is essential because we remove ourselves from the source of production in favor of expanding into terrestrial zones previously occupied by traditional livestock, which would be completely obsolete and wasteful at this point. This species would likely constitute a grain of some kind, one designed to withstand extremes in temperature and moisture content of the soil.
The third species would be a marine autotroph, possibly an algae of some kind to be used for human sustenance. This species would ideally be the backbone of our diets, with an emphasis on getting the producers to express essential vitamins. That is, we would apply techniques similar to those used to generate golden rice to stimulate in the algae the production of the most critical components of our diet. These would be the vitamins we would be unable to obtain by traditional agricultural practices, which would be obsolete in the near future as potable water becomes a more significant commodity.
We could be even more provocative, and utilize this last species also as a kind of ocean maintenance workhorse, collecting and depositing or integrating pollutants of great concern to our species (mercury or lead, for example). The harvesting process would first involve purifying and harvesting these pollutants, followed by packaging and distributing the edible matter to consumers. We could even attempt to generate a mechanism to promote the uptake of iodine (present as the anion iodide in seawater) to have this species mimic the effects of iodized salt, meeting yet another essential mineral need in humans.
All members of the triad would have a network of fail-safe mechanisms to prevent contamination or overpopulation, preserving large numbers of species that would otherwise be overwhelmed and out-competed by our highly favored, custom-tailored organisms. I would warn that making these mechanisms too easy to trigger would only increase the probability that a cataclysmic tragedy or act of hostility would leave us without one of our primaries.
Synergy between members of the triad would be of the utmost importance, as would constant monitoring of the ecosystems to prevent the loss of an essential, yet relatively unknown or underestimated member of the biotic community. Vigilance would be the price of endless sustenance, as the area available for agricultural use by humans increases by an order of magnitude, to say nothing of the existing farmlands that could be used to house many billions of individuals.
