If Cena is around for 30yrs and is still relevant then we'll talk. If Cena can somehow re-appeal to every demographic then you could compare him but i don't see that ever happening again.
Hogan appealed to every demographic tho he was aimed at families and kids, he had the whole world behind him (well seemingly) and is still a household name that will never be forgotten few other people in the business will ever have that honor other then by a wrestling fan, Cena will be forgotten within a few years of him moving on IMO.
Gotta give Cena a break in one respect tho, he's got far more talent in the ring when he does go, as a character he's just stale and lame thats my only issue not his "five moves of doom" i got over that years ago. Hogan had the presence/charisma that overshadowed his inability to actually perform in the ring Cena does not unless you are female or a kid.
as for what he's done in the time he's been in WWE, Cena has done more in that time then Hogan did in the 9yrs that he was initially in WWE IMO other then have a Hogan vs Andre WrestleMania moment which was a truly Global Match. sorry Rock vs Cena just doesn't cut it. Rock vs Hogan absolutely
So yes you can compare him in the fact that both have carried the company almost soley on there backs through tough times and yes both are PR dreams, Both have been in movies and stunk, both had a heavy kid following, both have held alot of titles and both have main evented Mannia more often then not BUT, Cena will never be Hulk Hogan nor Ric Flair
