I liked Cena's rap gimmick. I mean, the Wigger thing was sort of annoying, but as far as cutting promos on his opponents, it was literally the only time his character had any real edge at all on a consistent basis and it's what helped get him so mega popular in the first place (this was before the John Cena we know today that gets booed mercilessly by most of the audience as soon as his music hits.). Of course his raps got cheesy around mid/late 2004 or so (although that didn't halt his rise in popularity), but that's a different issue.
Anyway, as for The Shield;
Dean Ambrose will play the role of an arrogant and unstable sort of psychotic individual who is unpredictable and could snap on his opponents at any moment. I imagine a cross between Jake "The Snake" Roberts and Brian Pillman and maybe a little bit of "Psycho" Sid as well.
Seth Rollins will be the Shawn Michaels of his generation, maybe. (Ok, technically, Daniel Bryan has that spot sort of locked down, but Rollins' size, agility, quickness and selling makes for a strong comparison with HBK as well.)
Roman Reigns will be the next Batista. Large guy with a certain aura around him that will get over huge due to good booking despite his decent-at-best mic skills.