It isn’t mentioned often, but there is another dimension to the nuclear threat that could become real within 10-20 years — miniaturization of nuclear weapons continuing to the point where a nuclear weapon consists of several UAVs that converge on a location, assemble into a complete bomb, and detonate. You could use redundancy to ameliorate the risk of one of the UAVs getting shot down.

There are numerous strategic/military advantages which give this weapon a high probability of eventual development. Obviously, you would avoid using a missile, which shows up pretty definitively on a radar screen. For a first strike, this is tremendously important. Another advantage could be self-detonation in the event of discovery, something difficult to implement with conventional missiles.

Update: this technology would have a significant advantage over using UAVs alone because the warhead that could fit on a single UAV would have to be very small, and would have frustratingly low yield. A warhead built from converging components could have arbitrary yield, while retaining the stealth benefits of UAVs.