Since Pro Wrestling is scripted, then the kayfabe accomplishments shouldn't matter at all.
Eight years reign as WWE Champion (Bruno Sammartino), 4 years reign as WWE Champion (Hulk Hogan), the longest WWE Champion in modern era (CM Punk), 16 World Title reigns (Ric Flair), 15 year unbeaten Strek (Andre The Giant) and even Wrestlemania Streak (The Undertaker) should never be taken into account to determine the Legendary of a wrestler.
Instead, the non kayfabe accomplishments should be the ones which matter. The Legendary career would include the Legendary feuds, moments, and matches the wrestler had in his career. Look at these Legends for examples : Stone Cold, The Undertaker, Triple H, Mick Foley, The Rock, Bret Hart, and Shawn Michaels. These Legends had Legendary careers for sure.
Everyone would talk about these seven wrestlers till at least 50 years from now. Look at Stone Cold. Even after he long retired, many of IWC members still talk about him everyday. Same with The Rock, Bret Hart, and Shawn Michaels. Also, The Undertaker, it's almost everyday IWC questions when he will return.
Other than that, wrestler such as Edge could also be added to the list. Everyone still talks about TLC matches he had, his funny segments with Christian, etc. And Eddie Guerrero.
I would like to add Ric Flair, Hulk Hogan, Andre The Giant, Terry Funk, etc, but these wrestlers are before my time.
A wrestler with a Legendary career include so many aspects including great gimmick. But, the most important thing is how much Fans will talk and remember you after you retired. That's the most important thing. It's like Muhammad Ali and Mike Tyson in boxing. Boxing Fans keep talking about them, even after they're long gone. But, the difference is boxing is a real sport, therefore we judge them on their accomplishments. But, with wrestling, it's all the great Legendary feuds, matches, and moments the wrestlers have shared with us in his career.
BQ : In WWE, today, no, it isn't possible at all. But, back in early-mid 90s (before 1997) and Ruthless Agression Era, yeah it's very possible. Look, Chris Benoit did that without a gimmick at all. Benoit just relied on his wrestling skill. Then look at Bret Hart who doesn't have great gimmick. Bret Hart's only known as a superior technical wrestler, not as a gimmicky wrestler.
And wrestlers such as Shelton Benjamin who didn't have great gimmick at all also got pushed back in 2004. Too bad, he never makes to main event status.
I strongly disagree with anyone who sad John Cena isn't a gimmick. Cena is a gimmick. He played a babyface superhero who always stands for what's right, never backs down from any challenge, never gives up, and plays fair by the rules. I don't believe at all that Cena's like that in real life. His betrayal and divorce of his wife says it all. His wife said Cena cheated on her with Mickie James. That already shows that Cena isn't a good guy we saw on TV in real life. If Cena always stands for what's right and never backs down from any challenge, then he wouldn't cheat on his wife and even divorce her. And we also hear rumors that Cena is the one causing Kenny Dykstra get fired. I find it's very possible that one day WWE Fans would find out that Cena's a jerk in real life, just like Hulk Hogan was.
Other than those two eras, it isn't just possible at all to be successful without great gimmick. WWE won't push anyone without a great gimmick.