I am interested of a fictional future world where people can walk into a bar, order a drink with the flavors they want, and as that drink flows through their system, their DNA is modified.
An example of desired DNA modification is, let's say, the electric eel. More than four-fifths of the 2 meter long body of this fish consists of special battery-like cells that can collectively deliver a jolt of up to 600 volts. The interesting thing is that the eel can completely paralyze a nearby prey by releasing this stored energy as an intense volley of around 400 high-voltage pulses per second. (This information is from this National Geographic article.)
In this fictional world, science has advanced sufficiently far to know which DNA code needs to be in place to allow humans to use their arms as the battery (the arms would replace the four-fifths of the eels body) and how to add the needed extra functionality into the nerve system, etc.
So the knowledge of DNA code and gene-phenotype interactions needs to advance to that level.
What technological advances would need to be in place to make it possible for that civilization to produce drinks that can contain the DNA delivery vehicle?
Does it require nanobots in the drink? Or can a virus be encapsulated so that it survives the stomach acids? How could the immune system be bypassed?