As a piece of fantasy booking, it's a fun exercise. In reality, no way.
Stone Cold appears every now and then and when he does, it's an attraction. It should have been that way with The Rock too, but with all the furore around his promising to never leave then satellite/sporadic appearances, he doesn't seem to have the same backing.
For these guys to wrestle at WrestleMania, they need TV time. Rock - Cena has been kept relevant throughout the year thanks to Cena, and to an extent Punk, mentioning Rock's non-appearance after his return just over a year ago now. Rock will appear more now in the run up to April 1st but it's an active superstar who has kept this feud bubbling away, along with a big 4 PPV appearance at Survivor Series.
What I'm getting at is were Austin and Rock to suddenly feud, they'd have to both make time for it to appear on TV. While they can use social media to perhaps go back and forth with each other, it's what's on TV that matters. They could call each other whatever they wanted on twitter, post videos of them verbally tearing into each other, but the payoff wont be on social media. It needs to be on PPV.
Let's go further back to Rock and Austin's match at WM 19. The story was Austin had got the better of Rock at their last 2 WM matches, and Rock had to beat Austin to prove that he could do it on the biggest stage to satisfy his ego. Austin still leads Rock 2-1 at Mania, one more match could tie it up or have Austin go 2 clear. If Rock ties it up, would Vince/WWE really refuse the chance to coax one more match out of the 2 to see who is the better man? That means more commitment from Austin and Rock and more time focused on older stars rather than new ones.
These 2 guys are massive names who did a great deal for the business, but it's now up to the current crop of guys to spark some kind of magic, not have time taken away from them by 2 veterans who have expanded into new ventures outside of wrestling. Would it be great to see them go at it again? Dam right. But should they at the expense of the current guys? No.