To me a world war will entail large powers directly fighting each other on multiple fronts. Nukes don't have to be involved but could be.
Something like:
Russia, China, Iran, Iraq, N. Korea Syria and some of their allied countries (Cambodia, Cuba, Pakistan, Afghanistan, Lebanon, Venezuela, Ecuador, Argentina, Mongolia, Belarus, Nicaragua...)
against
America, NATO, Israel, Japan, India, Australia, Philippines, S. Korea and some of our allied countries (Columbia, Mexico, Saudi Arabia, Jordan, Vietnam, Thailand, Honduras, Chile...)
My list of allies are based on current politics and who individual countries are friendly/enemies with. Such as Argentina and England not playing well together, Iran's growing hold over Iraq and India joining America, Japan and Australia in the S. China Sea to contain their biggest rival China.
Notable neutral countries: Brazil and S. Africa.
Some possible fronts: Pacific Front, Eastern European Front, Middle East Front, South/Central American Front, North African Front.