First of all, I define a World War as a conflict that spans across many different nations stretching across the world. Therefore, by this definition, the War on Terror was a world war, since it involved Iraq, Afghanistan, the United States, the United Kingdom, Pakistan, Yemen, the Philippines, etcetera. There were tons of other nations involved as well.
But if you are referring to a future war, I do predict a future Islamic takeover of Europe and maybe even the formation of an ultra-fundamentalist Islamic Caliphate, Or perhaps China would create a hegemony over nearby nations, this "Iron Curtain" being rivals being certain Western nations, in a sense a second Cold War.
With tensions rising between the US and North Korea there is sure to be a war. Or with China, who we seem to be trusting less and less. How can someone think just because the UN says no war that countries will follow? And if there is a US vs North Korea war our (US) allies will go up against North Korea and their allies (including Syria, Russia, and maybe China). It won't be pretty
Life is getting pretty boring and there isn't shit all on tv. I'm bored. I want to see a war break out. I want to see foreign troops marching through cities beheading men and raping women. Yeah! Fuck humanity. Look at all the bullshit everyone is obsessed with now days All this celebrity and technology shit. It's time for a reset. I hope a couple nukes go off as well. Clear out some of those over populated landscapes. Cross my fingers hope you die. World War 3? Yes please
We will clash and it might be because of oil or maybe North Korea might get some bright ideas. It can even be about The Spratly Islands in Asia. China, Vietnam, Japan, Taiwan and a whole bunch of others are on tight waters now. But right now, Syria might be the leading cause of a WWIII if the UN decides to join the fight.
It may not happen in our lifetimes, or this century, or this millennium (though that's unlikely), but it will almost certainly happen at some point in the future. The only way it could not occur is if the human race becomes extinct before World War 3 can break out, and that's even less likely.
Even if world war 3 could possibly happen it would have to be far in the future. The reason the first 2 world wars were fought were over power an who could be the next super power, but now the undeniably super power (unipower) is america plain and clear. No other country has a military to compete and are all to economically interdependant with america and if there was a war, economies everywhere would collapse. Also we now know the sheer danger and power of nuclear weapons and no one wants to EVER use them against each other again.
You would not see a massive "everyone takes a side" style war any longer, due to the intricate nature of alliances in the 21st century. There is of course also the matter of nuclear capability, complicated debt issues, and the existence of international bodies like the UN. "World War III" is so unlikely as to be dismissible.