After years experience as a GM, I've learned that when I set up a campaign, I take a little time to integrate some tailored antagonists for the purpose of using them should a player take up some disadvantage like "Enemy". These antagonists are also set up to show anyway as the favored hench of a boss or such, thus they can multitask withut interrupting the plot.
Takes a little work to set up, but the investment is worth it.
Hilarious experience point
--Once I was a player in a Shadowrun game and everyone had taken an enemy disadvantage except me. The GM had a bit of a headscratcher because he wasn't sure how to work this without giving several players the same enemy or not giving the enemy any "screen time". I helped him by providing an interesting solution--
All the enemies were corporate investors from different Megacorps that met at a party. In conversation they learned that they each had a particular runner they hated and wanted dead. They all decided to make a betting pool: Winner was whoever killed off their runner first.
Thus, we occasionally had Johnsons from competing enemies give us jobs tailored to kill their specific runner (But couldn't kill the other runners because that would constitute losing the bet). For example, the enemy belonging to the Hacker would have the Johnson give us a data-steal job and ensure there was some extra Black IC in the node to fry his brain. The Street-Sam enemy would tell the factory guards to aim for the big guy first when the Runners show up to sabotage the facility.
A little complex, but it worked to make the disadvantage playable.
And of course, *I* ended up being killed first.