Never say never. The last two happened with less than a year's warning. It is not wise to underestimate the human capacity for monumental stupidity. That said, I do not see a "world war" happening in the near future, simply because, as you say, the only powers capable of launching a world war (the US, Europe, Russia, China) are so economically dependent on each other that to destroy one is to ruin all; a kind of economic MAD (MEAD?). However, a small-scale nuclear exchange between the mid-tier members of the nuke club is a genuine possibility. India and Pakistan have been gleefully disputing the relevance of MAD to their situation (one Indian general said, and I'm paraphrasing, "We kill 200 million of them, we've wiped them out; they kill 200 million of us, we have a billion left") while Israel and Iran seem locked into their own sets of apocalyptic delusions.
The most terrifying scenario is Pakistan falling to Qaeda-sympathetic Taliban-style government. It is almost impossible to gauge how likely this is, because events that would augur political collapse in any other country are a slow news day in Pakistan. Were that to happen, the War on Terror would immediately jack up several notches, and Russia and China, already peeved about the too-interventionist response to the Arab Spring, would doubtless start to bristle.
A nuclear war between Israel and Iran is not an immediate threat, simply because, while Israel has nukes, Iran currently does not. The prophesied attack by Israel on Iran will probably happen before 2015 or so, but, analysts agree, would be unlikely to stop Iran's developing a nuclear weapon. So sooner or later, there will be a nuclear balance of terror in the Middle East. Whether both sides, faced with the immediate prospect of self-annihilation, will abandon their apocalyptic, fire and brimstone rhetoric and adopt a pragmatic approach remains to be seen. If their deeds instead match their words, the prospects are pretty grim. While the long-predicted nuclear war between India and Pakistan would likely have limited (for a given value of "limited") global impact, a nuclear war between Israel and Iran has the potential of irradiating and thus rendering useless roughly 80 percent of the world's oil reserves, an effect that would change world history, some might argue for the better.
A far more hypothetical threat is how long the US will continue to tolerate China's stanglehold on its debts. Some have argued that, with its consumption rates rising and arable land decreasing worldwide, China might instigate a "debt for grain" agreement with the US, which would see prices for staples rise in US supermarkets for the first time in history. Right now, the US is seeing its economic supremacy challenged by China, and its political supremacy challenged by the BRICS bloc. Its one remaining trump card is its overwhelming military supremacy, and as Fareed Zakaria said, "to a man with only a hammer, every problem starts to look like a nail".