The Russia 2018 World Cup is loaded with fascinating plot lines on and off the pitch. Names like Cristiano Ronaldo, Lionel Messi and Mohamed Salah lead a chorus of superstars striving to be crowned the kings of football.

Five-time winner Brazil wants the trophy back, while Germany is trying to become the first repeat champion since 1962. Italy, the Netherlands and the United States are all watching from home, while World Cup first-timers Iceland and Panama look to make an impact.

The host nation, meanwhile, has emerged as perhaps the tournament's most controversial site since Argentina's military regime hosted in 1978.

All sorts of questions have emerged ahead of the World Cup launch Thursday.

Now or never for Messi and Ronaldo

With Ronaldo at 33 and Messi turning 31 this month, there is a feeling that both superstars won't get a better chance to capture the one major title that has evaded them.

True, this generation of elite athletes -- from Rafael Nadal and Roger Federer, to Tom Brady and LeBron James -- is changing the rules of longevity.

But even if Messi and Ronaldo return for a farewell at Qatar 2022, the chances of either having a supporting cast of equal strength is low.

Ronaldo's Portugal is ranked world No. 4 by FIFA, coming off the high of winning Euro 2016, and aided by the emergence of 23-year-old Manchester City midfielder Bernardo Silva.

Messi's Argentina is just a notch behind at No. 5, returning strengthened after losing by an extra-time goal against Germany in 2014 final.

Messi will operate the attack with veteran marksmen Sergio Aguero and Gonzalo Higuain -- both in their third World Cups -- and emerging 24-year-old Paulo Dybala.

A World Cup win for either Messi or Ronaldo could finally put to rest the debate of which of the two is the greatest ever. Or would it (CNN)