A lot of people say Roman Reigns is "Samoan Cena" but I would actually argue that WWE is intending to make Roman the "new Cena" for Cena fans as they get older. Basically when Cena fans grow to become angsty teens who no longer want to wear bright colors, in comes Roman Reigns. His character has some similarities to Cena but a few key differences, mostly in appearance:
1) He's not wearing bright colors, but instead blacks and dark blue
2) He has tattoos
3) He has long hair and facial hair
4) He wears body armor
5) He more openly defies the authority and goes against the grain
He is supposed to appeal to the Cena fan that has developed into the "rebellious teen". He doesn't come from the ramp like everyone else, he comes through the crowd to be "different". He doesn't give hugs to fans, he gives fist bumps. He's not happy-go-lucky, he's brooding. He doesn't come out a talk for 20 minutes; he punches first and asks questions later (most of the time). He sometimes says "edgy" things like "bitch" even if it is part of the same super babyface promo.
It's really just a repackaging, though. He always wins just like Cena, but it is presented more as dominance than overcoming the odds. One Versus All is just Roman's way of saying he'll never give up. Just like Cena, he will never sell out.
I think the WWE had a similar transition from kids to teens in the 90s. Bret Hart was hugely popular with kids, but was phased out as the face of the company for Shawn Michaels, who had a more juvenile character. Michaels would then be phased out (partially due to injury obviously) for Austin to step up as the fans wanted true edge instead of juvenile jokes.
TL;DR: Roman Reigns is basically PG-13 Cena