Uh, as someone said it, Heath Slater. He'd easily become one of the most hated heels back then.
Also, Tapout asked whether Roddy Piper would have gotten over if he debuted now, and my answer is NO. He'd end up as a midcard champ, but would never get to the main event. He'd end up jobbing to Kane at the end of the day, if he was debuting now.
Now, back to the whole Heath Slater thing. He's one of those Miz-like guys. Why Mizlike, well, they are those loudmouth heels with trademark catchphrases that make people boo the soul out of their bodies, if you get me. I can hear him yell I'M THE ONE MAN BAND BABY about 13 years ago and I imagine all the heat he'd get, and the same goes for Miz. Actually, Miz would be one of those heels that get cheered, if you ask me.
Another thing I've seen in this thread is that somebody mentioned Del Rio. Del Rio would have gotten over during the Ruthless Aggression era, or maybe at the end of it. Around 2003-2004, he'd be big. I could imagine him feud with guys like Benoit (imagine the match), or with the Evolution guys, maybe even Lesnar. Or Eddie, yeah with Eddie from those days. Remember the whole Eddie vs Chavo feud. The Guerreros were pretty over back then. I could see him feud with them. There are a lot of possibilities. Think about it.
As for Ziggler, Ziggler would have gotten over in the Attitude era, but with a slightly different gimmick. Less show off things and such, although I personally love the whole Show Off #HEEL thing.