It's my opinion that Cena is this generations Stone Cold, or more accurately the Rock. Cena, since his first face turn, has never been heel, and probably will never get the chance. He's the biggest draw right now in the entire company. Now imagine John Cena never came to the WWE...? Boy would things be different. Besides being the top draw, he's the top face in the company. Guys like Triple H and Jeff Hardy are fan favorites, but are often too conflicted to be pure face characters. Here's the things I think would be different if John Cena never came to the WWE.
-WWE would have pushed a lot harder to keep guys like Kurt Angle, Christian, and Booker T around. Look at the better part of the last five years. JBL and Cena had a really long epic battle. Edge and Cena have had one of the best feuds in history over time. Cena/Orton, Cena/Jericho, Cena/Jericho II, Cena/Batista, Cena/JBL, Cena/Edge, Cena/blah blah blah, etc. Who would fill all those matches and feuds? Booker T, Kurt Angle, Christian, Jeff Hardy, Rhino, etc. Those are the guys who could have stepped up and filled those matches. They may not have been as good, but they would have had to. With that being said, TNA wouldn't even be close to being what it is today. If WWE was forced to hang on to those talents, TNA would be dead before it started.
-Mr. Kennedy would be pushed like crap. WWE would try to turn him into the top face
-Matt Hardy and Jeff Hardy would be forced to step up their mic game.
-Randy Orton would have been forced into his position sooner.
-Shawn Michaels and Ric Flair and Rey Mysterio would probably have at least one more title run
-The WWE would have tried harder on smaller talents that they let go far before they had a chance to prove themselves.
-The WWE would have a huge lack of good promos. Edge, Y2J, and HBK would pretty much be where it's at.
So with all that being said, I think trying to imagine a WWE without John Cena is trying to imagine a WWF without Triple H or Stone Cold. It's like trying to imagine a WCW without Booker T or Sting. It's not that those people carried the company, but they balanced it out. The Rock was nothing without Stone Cold. DX was nothing without Triple H. WCW was nothing without the Five time, five time, five time...or the great phenom of Sting.
I think you'll see in a few years that guys like Edge and Randy Orton were made into the heels they are because of John Cena.
I think the WWE would be pretty much screwed without Cena. He's the top star of this generation. Orton comes next and Jericho is right up there. But guys like HBK, Triple H, the Undertaker, etc have been around forever, and sooner or later you get tired of them doing the same thing for two decades.
Without John Cena, TNA would be dead, Edge wouldn't be the greatest heel of this day, the crowd would never have had anybody to cheer and boo at the same time for... Honestly I might not be watching WWE right now had it not been for John Cena. I have almost given up on the brand three times. I stayed once because of the Summerslam TLC match with Cena/Edge, once for Wrestlemania 21 with Cena/JBL, and was finally convinced after last years Wrestlemania that the WWE was headed in the right direction...so who knows. Those are just my thoughts.