Well WWE hasn't built up enough of their future superstars so it might take a lot of buildup just for them to start main eventing. I think superstars like Jack Swagger, Evan Bourne, John Cena, Randy Orton, Edge, Matt Hardy, John Morrison, and MVP main eventing in the future.
Jack Swagger is a great wrestler and he has decent mic skills and charisma. Jack Swagger already gets a huge amount of heat from the fans and he's probably the best wrestler on Raw next to Shawn Michaels. Jack Swagger has the size, speed, and power to go far. Swagger also wrestles with the safe technical mat wrestling style so i see him main eventing in the future for a long time. Wrestlers like Evan Bourne will be the main high flier of the company and John Morrison, MVP, Dolph Ziggler, and Matt Hardy will eventually fight for world championships when the older wrestlers such as Undertaker, HBK, and Triple H retire.
John Cena, Randy Orton, Edge, CM Punk and other long time main eventers who are still pretty young will always be pushed by WWE for a long time and they'll be the ones to help out the younger guys and fight for championships. I see John Cena being the top face, and Edge going back and forth as a face and heel, and Orton will most likely continue the run he is going now. All these guys have great charisma, make money, and bring in ratings so i think these superstars will be the big ones such as HHH, HBK, and Undertaker in the future.
The future of TNA looks bright to me and unlike the answers above i don't think Kurt Angle will be main eventing in TNA for 5 more years. I see some of the younger superstars such as Alex Shelley, Chris Sabin, Samoa Joe, AJ Styles, Matt Morgan, and some of the X division wrestlers along with some of the main eventers such as Abyss. TNA has built up their future superstars more than WWE and they are all solid wrestlers in the ring. TNA also has better tag team and knockouts division so unless WWE does something i see TNA overshadowing them in those divisions.
So basically in WWE:
Jeff Hardy (if he decides to return)
Some of the X Division superstars
Those are the main eventers of the future in my opinion for both promotions.
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