Undertaker
John Cena
CM Punk
Randy Orton
HHH
Big Show
Edge
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7 Established main eventers
Kane
Rey Misterio
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2 Veterans who have main evented
The Miz
Wade Barrett
Sheamus
Jack Swagger
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Four recently promoted main eventers
John Morrison
Dolph Ziggler
Daniel Bryan
Mason Riley
Alberto Del Rio
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5 breakout stars who seem destined for that next level.
HBK
Chris Jericho
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2 main event level wrestlers who are still under some form of contract. If worse came to worse, I could see both of these guys coming back. If for example Cena, Orton, Taker, Edge, and HHH were all put on the shelf at the same time and WWE was really in jeopardy, these two are old fashioned enough to come lend a hand.
As far as when all of these guys could be gone, I mean some are in their 20s and early 30s. Depending on level of commitment, injuries, etc, it is reasonable to see them for 15 to 20 more years. I personally watched my first live televised Flair match in the 80s and my last one in 2010. It is possible.I'm going by the logic that these are names that will draw PPV revenue and not what the WWE wants us to perceive as main eventers. I'm going to eliminate anybody I consider a static main eventer as well like Kane, Big Show, Khali, Henry, or Chris Jericho. They have the potential to be main event, but they're not always main event.
John Cena,Randy Orton, Triple H (until he officially announces retirement), Edge (although he's borderline on the card flux list), Rey Mysterio, the Undertaker
BA: As soon as these guys decide to stop wrestling. It's going to be awhile before we have a world of wrestling without Edge, Orton or Cena (barring incidents that could jeopardize their career), but guys like Trips, Taker and Mysterio already have one foot out the door considering their lucrative future elsewhere in the company, health issues, or a sheer want to go back to Mexico.
As for future or up-and-coming main eventers of tomorrow (echo effect), CM Punk, Alberto Del Rio, The Miz, Wade Barrett, Dolph Ziggler, MAYBE Mason Ryan but it's too soon to say, Cody Rhodes, and that's about it in my opinion. You may have guys like Morrison or Sheamus as strong contenders to future world title holders, but right now I think the men listed above are going to be the ones drawing in new wrestling fans of tomorrow. They've got the uniqueness, the new way style, the charisma that plays off of the WWE writing system well (which is hard to find these days), the look in some cases, and most fans interests in general as things stand. I think any or all of these guys could play either side of the coin well, be it heel or face except for maybe Cody and still garner the heat and interest they already have today
Edge
Big Show
Undertaker
Kane
John Cena (sadly)
Randy Orton
Rey Mysterio
Jack Swagger
Dolph Ziggler (sadly)
The Miz (sadly)
Jack Swagger
Wade Barrett
CM Punk
Triple H
Sheamus
John Morrison
Alberto Del Rio