I would say N. Korea may pose a threat to the US in the future if they ever manage to gain nuclear weapons
North Korea has nuclear weapons. Having them doesn't make North Korea a threat, although, the possibility of someone else getting them from North Korea maybe. North Korea is all about posturing, but they understand if they ever launched a nuke at anyone their entire country would be wiped out just as soon as anyone realized their nuke was in the air.
I think if Iran gets nukes it is even less threatening than the fact that Pakistan already has them. Iran is a fairly stable country, not too many crazy militants threatening the government too often. Basically the government in Iran, i.e. the Grand Ayatollah Ali Khamenei, has a great grip on things in his country, with a strong army and reasonable plans for economic development.
Pakistan on the other hand, who happens to be our ally, has nuclear weapons, a disjointed and weak government, a powerful military, the ISS, basically running things and strong-arming the president. They have Taliban in their tribal areas, and attack from India to worry about. Their economic situation is just terrible, and there is a large possibility that through this instability they could lose control of their weapons. Thus, it would seem our ally with nuclear weapons and less stability poses more of a threat than our supposed enemy Iran, even if they have nuclear weapons.
Although, I will grant that the U.S. has a special, secret program, with hundreds of millions of dollars dedicated to it to protect Pakistan's nukes, so eh.
Back to the original question. Communism is not a threat to the U.S. unless somehow Russia manages to gain a great deal of influence in the middle east...and we all know what happened the last time they did that. Russia could also possibly try and do the Soviet Union thing again but fortunately for the rest of the world, most countries that were a part of it, are no longer interested in being a part of it.
China is probably the biggest conventional threat. They have a booming economy, massive army, they are able to manipulate the value of their currency to their benefit on the world market. As well as an excellent technological sector that is getting better as they get richer.
Lastly, terrorists are the biggest current threat to the U.S., unless of course, you consider the way the U.S. has been being run by our leaders for the past...well ever since Reagan took over.