In the presence of 18th amendment, federal government has no solution for Karachi. Even Governor rule is not an option because, as per 18th amendment, Governor rule has to be endorsed by relevant provincial assembly within 120 days of its imposition. Otherwise, it will become void after 120 days.
When PPP, ANP, and Balochistan parties were working minutely on the drafting of 18th amendment, GHQ was sleeping while PMLN was focused entirely on removing the ban on 3rd time PM ship ( so that NS could become PM again).