Stone Cold Steve Austin!
Even though he made his impact in the little main event time he had, there was still potential to do so much more with other superstars. If he were still wrestling in 2003/2004 full time, we could have got a match with Goldberg. Wouldn't have been as big of a deal as it would have been in 1998, but still. We could have got an ACTUAL feud and match with Brock Lesnar the way it should have been done, as opposed to the way WWE tried to go about it in 2002. Remember Brock stealing Austin's four wheeler and Austin going to Smackdown to reclaim it in 2004? It was awesome.
Others he could have clashed with:
John Cena - Kinda writes itself, though I feel Cena/Rock probably had better chemistry than Austin/Cena would have, just like Rock/Hogan did, strangely.
Batista - Imagine Austin being unimpressed with Batista man handling those smaller than him (which is most people) and saying he'll still "drop Batista's big 300 pound jacked up ass on his damn chin with the Stone Cold Stunner."
Randy Orton - The Legend Killer trying to make a name for himself by going after none other than the Texas Rattlesnake. These two would have the best chemistry if Orton was The Viper when he clashed with Austin, but unfortunately, Austin would have likely been retired by that point. Orton trying to kill WWE's biggest legend ever opposite of Hulk Hogan to make a name for himself is still a logical enough reason to feud.
Edge - the Rated R superstar saying he's not only more controversial but better in the ring than the redneck anti-authority hero. I could see these two having a pretty good brawl. I also see Lita slapping Austin (imagine the feud took place in 2006) and getting a stunner for it. As if those chair shots she took to the back in 2001 wasn't enough.
JBL - These two had a beer drinking contest in 2006 that was somewhat entertaining. They also went at it briefly with fists and a Clothesline From Hell and a Stone Cold Stunner at Cyber Sunday 2008. Would have been somewhat entertaining. Austin definitely wins this feud, of course.
CM Punk - This could have happened already when Punk was working his Straight Edge Society gimmick. Imagine him preaching against the evils of Austin's beer drinking and finger jestering and cursing and trying to say he could do what no other can do, and that's "save" Steve Austin. Austin responds that Punk is the one who will need saving when he opens up the biggest can of whoop ass anyone has ever seen.