Good post, while i may be repeating a lot of what has already been said, I do agree as a Fan who is sick of seeing him at the Center of almost everything for the past few years that a break wouldn't be a bad thing.
However...I don't know if now is a good time to do it, clearly the kids still love him, (even though a lot of them turned on him when him and Rock faced off) they still love him, some adults too, and as someone said when only 10% of most crowds hate him most of the time, WWE would be taking a risk. But this Thread as you said was about the Character stand point, and not how much merchandise etc he sells.
YES, he needs a break, People have mentioned how Rock had and Injury, Taker, Austin, HHH have had an injury over the years which helped them by having forced breaks to heal and come back stronger then ever, While Cena has had a few injuries, he hasn't missed a lot of time overall, and he is fortunate to be in that position. But another wrestler i can think of who has been in exactly that position is Chris Jericho, (before i continue i don't won't to hear how Jericho isn't as big a star etc as Cena, he is), But even Jericho decided to step away from awhile. To focus on other things, to Just have the break and get the creative juices flowing again, so-to-speak, and it worked, Jericho came back and gave us some of his best work ever, feuds with HBK, tag tea with Big Show etc, alot of it had to do with him feeling refreshed, as he said in his book, it just gave him time to watch as a fan again, and decided to get involved again, Jericho has always been one of the most talented guys on the roster, even if he didn't take a break I'm sure he would have succeeded one way or another progressing himself. But even he realized that eventually you just need to step back, a) for yourself and b) to allow others to step up, so if/when you do return you have some fresh talent and big names to work with.
Cena needs to take this approach, he can still do a lot behind the scenes, signings, appearances etc, maybe do another movie for 4 months (even though his movies weren't the best, but at least he could focus energy elsewhere and be creative in different ways), A missed opportunity though was when he got 'Fired' recently, i liked where that storyline was heading, but then he just showed up every week anyway, i mean i get the whole 'Never Give Up' attitude but seriously is again just made everything leading up to that story look weak, and sorry to say that has been a lot of Cena's stories as of late, start out strong and end up weak, usually by a Joke promo where he kinda buries his opponents by not taking them seriously and ruining whatever strength they had in the first place. Wade Barrett is a good example, was built up as a big threat, but then now what, after the eventual Cena defeat, goes to smackdown to job to Kane and Big Show. (again i know that isn't the full story but essentially what happened).
One thing i don't get, is WHAT exactly is Cena's character as of late? Everyone says he has that Whole Never Give Up Attitude which is great to have, or the Hustle Loyalty Respect thing which he has had for years, and basically the Super Cena we often see, but i really don't get just what his character is. Undertaker is a character, HHH is character, CM Punk is a character, you know their motives their drive, their intentions, Cena doesn't really show this much, as lately he is just a target of everyone else, CM Punk, Miz, Rock, i don't see much of a character sorry to say. Cena is just there, and because of his name value seems to be just involved and Kids seem to follow it but as a fan of wrestling for almost 18 years, i just don't get Cena anymore character wise.
He has even out shadowed the Championship to a degree, i mean look at it this way, on Smackdown the writers a lot of the time, make the Belt seem like a BIG deal, as it should, on Raw overall the belt is just a background to the Cena stories, and as big as Cena is, that's not the way it should be, case in Point Cena did nothing really to make Miz look like a threat going into mania, and nothing to make it seem like he was really after the Belt. I think this is partly due to the stale Cena character, but rest assured he will be after the belt again soon, but why? Everyone is after i get that, but if Cena continues to stay around without a break WWE needs to take him from the title picture for a year, maybe 2, his character would not suffer that much, put him in some big feuds, create new enemies, maybe even do a storyline where a lot of the heels have decided they are sick of him winning the belt and continuously screw him over the months to keep him out of the title picture so they get their shot, this creates new enemies for Cena and gives other talent a chance to elevate themselves and try to create new stars.
Let's be honest, over the last few years how many times has Cena been involved in a Championship match, too much, while some argue there is no other talent that belongs in the title picture, that is partly WWE's fault for not elevating new talent, and also not using the talent they have, Morrison has been ready for as long as Miz has, yet he hasn't really had a big PPV title match, not counting EC 2011. I'm talking singles competition. R-Truth is a excellent wrestler when given the chance, Christian, Dolph Ziggler, Sheamus again, Daniel Bryan (some might not agree, but if he won it, i don't see a problem), Kofi, Jack Swagger, Drew McIntyre, all these talents are worthy of title shots, when they are used properly, they don't have to win it straight away make them chase it, but there are plenty more options is all i'm saying then having Cena in the title picture almost every 2 months.
Anyway gone on a bit long, but a break would help Cena, it gives his fans a chance to miss him, and have him come back stronger, and hopefully with a bit more of a character, As much as i like many want to see him turn HEEL, i can't see that happening while he is getting cheered and selling merch like he does, it makes sense from a business standpoint, but not from a character one, i would like to see it, a good time to do it would possibly be around WM28 when Rock comes back, as we know he will most likely be booed, then why not have him use that, "You cheer Rock over me, yet i'm here all the time, Screw You" simple and effective it can evolve from that, and yes Rock may not be around after WM28 but at least it can start Cena on the heel path, make him relevant from as a character again, give him a fresh chance to change/grow and if he goes face again later so be it. But Cena is getting stale, and something does need to be done to freshen him up again.
