There could be another world war, but it would be nothing like the previous ones. It would be like asking if WWI and WWII are really comparable. Although they were both fought by men with guns and various other technologies, the actual advances between them mean there's very little besides the title of 'World War' to compare them.
A modern war would be fought with nuclear weapons definitely, and each side would spend most of their time trying to deflect and destroy the enemy nukes while making sure their own land on target.
Any ground fighting would be done in smaller countries, like Afghanistan or other Middle Eastern nations. One side would send their army in and the other would respond with their own, but in the end both sides would resort to firing ICBMs at each other.
The trick is remembering what exactly the goal in the WWs was, and what it would be again if another war of the type happened. The Nazis wanted to conquer the world, and not knowing the true fallout of nuclear weapons, had they had them would have undoubtedly used them to speed up victory for the axis forces. So if another WW happened with, say, China wanting to remove America from equation entirely, what exactly would stop them from just long-range bombing America into the dirt?
Wars are fought mostly to the technology level of the day, and nowadays you don't even have to look your enemy in the eye to wipe him off the face of the planet.