I’d agree with @pdworkin that the mass warfare seen in the 20th century is unlikely at any time in the near future. A nasty regional conflict involving WMDs is much more likely.
China, under it’s present government, is not a threat. In fact China is probably likely to assist the US if the PDRK starts anything. PRC is really only supporting PDRK as a counterweight to the US support of Taiwan.
If Pakistans present government falls to the Taliban, with their limited nuclear arsenal intact, they may start something with India or try to smuggle-bomb a western target (they lack long-range delivery capability). Again, expect US, PRC and India to crush them.
Iran is playing the nuclear card, IMHO, for internal consumption. The theocracy is extremely unpopular with younger people in the urban areas. The ayatollahs are doing this to “create” an external threat to propagandize internally. If they get too close, or actually test a device, expect a preemptive strike from somebody.
As the oil reserves in Saudi Arabia, Iran and the other Gulf states drain out, expect trouble as their social welfare programs and internal security erode. Without the petroleum to attract western interests, there could be long and nasty civil or regional wars.
Russia might make a bid to grab back some of the former Soviet states, they have shown this tendancy in Georgia recently. A regional issue unless the US, EU or PRC intervenes; unlikely. Russia is modernizing her military largely as a matter of internal self-respect and maybe to act as a regional “player” again. The traditional Russian geographic interests for the last 300+ years.
China could implode in civil war over the huge inequalities in wealth that have developed since abandoning communism (except in name). Again, the threat of nuclear weapons (with ICBM capability) in unknown hands. China has had traditional claims on territory in Vietnam, Laos, Burma, India and Pakistan. A more aggressive government might make a grab for those. Of course there is Taiwan also, more likely to be settled peacefully under some “special status” agreement.
There is the ongoing tensions between Israel and the neighbors. It will be the usual bickering and shooting unless the US completely backs away (unlikely in the short-term).
Expect the US to be backing down from unilateral superpower status. With the staggering national debt and increasing demands for domestic programs, the US is likely to start going the route of Great Britain in the mid 20th century. Mothballed CVNs, scrapped bombers and a return to isolationism. This will create a power vacuum that regional players will strive to fill. The EU may be forced to step up its military presence in areas. China will assert a sphere of influence in the western and southern Pacific. India will try to assert a naval-based authority in the Indian Ocean. Piracy is already becoming an issue, possibly leading to armed merchant vessels as prior to the 19th century.
There is a slight possibilty of armed skirmishes in the Arctic Ocean as nations grab for undersea resorces. Canada is currently building armed ice-breakers to back up her assertion of territorial offshore claims.
I apologize for grossly exceeding the scope of the question. Just my semi-educated guesses based on history and 30+ years of direct involvement.