If Cena was injured (and again I'll echo the sentiment of those before me: I don't want to wish injury on anyone) he'd appear Live Via Satellite on every Raw. We'd have at least 2 weeks of replays of the injury, 2 weeks of build up before his first talk via satellite, a month or more of promos from hospital, rehab, make-a-wish then the beginnings of building hype for his return.
Call me a cynic but nothing short of a major scandal or coma (let's hope not) would remove Cena from WWE programming these days. Never again will there be a 2008 Rumble moment involving Cena's surprise return.
In the days that follow TMZ posting pictures of Cena snorting coke off the wrong Bella's backside when he was meant to be granting wishes we would see the usual suspects rise to the occasion. Punk, Orton, Bryan, Jericho, Sheamus ADR would all nicely fill the gap and we'd probably see guys like Rhodes and The Miz in main events too. Unfortunately the absence of one Superstar, even The Champ, only leave one spot to fill.
Other than that I agree that it would be nice to see proper tag team and Divas divisions again and to see the up and comers get their shot but that could happen anyway, as PSOjedi said "You should blame Vince McMahon, Triple H, and the Creative Team for all these talented unused superstar, not John Cena."