The biggest thing I want to see from Cena is some change over time in his character. I'm not saying turn heel, but just...be affected by things that are happening around him. Like, back at Mania 28, during the build-up to his first match with the Rock, he would go on and on about how this match meant everything in the world to him, how he didn't know how he was going to go on if he lost, how he was basically ready to die if it meant beating the Rock....and then when he lost, the next night he went "Oh well, there's always next time."When Brock Lesnar shit-stomped him at Summerslam, he was gone for like a week before he came back, single-handedly wiped out the Wyatt Family so hard they broke up off-camera, and was throwing himself at Lesnar again like nothing happened. Both of those scenarios should have shaken him to his core-- John Cena's the invincible superhero, and then someone comes along and absolutely trucks him. As annoying as his inconsistent selling is in the ring, it's nothing compared to how little he sells the plot.
Again, I don't necessarily need Heel Cena in order to be interested in the guy. I just want to see him evolve and adapt over time. Daniel Bryan, for instance, went from soft-spoken indie guy who Michael Cole hated for no reason, to a deluded dork whose MitB cash-in filled him with false confidence, to half of a dysfunctional tag team who bickered like an old married couple, to populist anti-authoritarian revolutionary, to beloved folk hero. Randy Orton's been a snotty upstart brat, a cold-blooded psychopath, a loose-cannon antihero, a spoiled corporate golden boy, and about a half-dozen other things which make his character trajectory far more interesting to track than Cena's, and I don't even like Randy. Everyone around Cena has changed and grown and risen and fallen, some for the better and some for the worse, but Cena's always just been John Cena, stuck forever in 2005.
They have a perfect opportunity to make me care about the guy again in this Kevin Owens situation. Cena's pushing 40, and he's slipping down the card in favor of the Shield guys. He's suddenly facing all these guys from NXT who are already giving him a run for his money, and they're barely getting started in their careers. Sami Zayn nearly took him out in what Cena thought was going to just be a fun exhibition match. Adrian Neville had him dead to rights before Rusev ran in. And Kevin Owens just wrecked his shit. A year or so ago Cena cut a promo where he said something along the lines of "The future has to come through me," and now it's doing just that. The unbeatable superhero isn't as unbeatable as he used to be, and Cena doesn't know how to deal with that.
This would be the perfect opportunity to start showing some vulnerability and doubt in a guy who's been bulletproof for ages, ditch the goofy shtick and start throwing those stank-nasty lariats like he was in his match with Owens. Maybe as he grows more and more desperate to stay relevant, he starts making decisions he's not proud of, like falling in with the Authority (or Paul Heyman) to give him an advantage against the Roman Reigns' and Kevin Owens' of the world. Give the guy an arc where he actually goes through some peaks and valleys, show us what good Hustle Loyalty and Respect are when the guy across the ring from you is just going to German suplex you until you die. How does it affect him when he starts to doubt himself, when all the cancer kids are counting on him? Give me something, anything, to make me care what happens to this guy, as long as it's not another "heel cuts a worked-shoot promo where they try to bring out his 'dark side'" angle because that never works.
Will they do it? Probably not. But they could do it, and that's what I'm hoping happens. Anything that doesn't involve Cena AA'ing Kevin Owens off of an ambulance roof at Summerslam is all I'm hoping for, really.