...could Sting v.s Undertaker be the main event of WrestleMania? In my eyes it probably would be if it was made official.
Pretty much answered it yourself there. IF it came to be then it would undoubtably be the main event of a Wrestlemania. Let's be honest people have been talking about it forever (or when WCW went out of business, feels like forever in my head). It is one of what people refer to as "the last dream matches" and people would flock in droves to come see it, it'd be one year in recent memory Vince would have no trouble with views and buys, I know the kids may not buy into the phenomenon but guys who haven't watched for years etc.. surely would.
Uhh...no.
First of all, I STILL don't understand why Sting vs. Undertaker is supposed to be some kind of dream match. Because they both wear black clothing? Other than that, I don't see what's supposed to link the two wrestlers in any way. They wouldn't even work well together, and the match would be average at best.
The match probably wouldn't be all that, but as someone brought up in another thread Hogan and Rock, Hogan and Warrior, Goldberg and Lesnar were all poor matches, it's if the prior booking gets asses in seats, they can hate the match all they want as long as they came to see it really.
The way I have always viewed them is as being similar if outside the ring. They are/were the two paragons of respect in their respective companies. As much as you never hear people with a bad word against taker, I feel the same can be said with Sting honestly. It's like a wrestler match, the one match all the guys in the locker room would probably all mark out for. Their weak character similarities aside, it's the fact their careers have eluded each other until this point, that is what would make the match special.
Second of all, there is NO WAY Vince McMahon would let Sting come into his promotion and main-event Wrestlemania. NO WAY. Sting would have to put in a few years before that would ever happen.
I'd say there are a few situations where Vince is willing to overlook a non-homegrown talent coming in and dominating from the get-go, Goldberg for example. I think Vince respects some guys unequivocally like the takers and michaels, hence why they are off on legends contracts working effectively part-time. In my eyes he would respect a guy that has evaded his management his entire career and managed to be the monumental success that he is. This isn't the same guy who in Paul Heyman's words "stole Bret Hart's dream and bought himself an airplane", I think with his age Vince has learned to supress his ego and gain humility. I honestly believe if Sting entered now he could skip the early transitional period and go straight to the big times, after all he is 51, I don't think he could afford to wait too long anymore.
Finally, Sting has always said he would not work for Vince McMahon. Perhaps for a one time thing, and the opportunity to be remembered forever in the Hall of Fame, may make him change his mind (as it did Bret Hart), but to the best of my knowledge Sting is still holding strong on not working with Vince.
This is easily the most important factor. It's basically will he go back on his word, will he lose peer respect if he does? But in concordance with what you said about Bret, I'm sure if you had asked the guy if he'd ever be back in 1998 you'd have a blank stare and a slap to the face. But like I said Vince has changed, he's not out winning royal rumbles now, he here putting over nexus and other young guys. I think Sting could go back on his word and strike a deal with THIS Vince McMahon. All Sting effectively needs is one run, 21/2/11 up until mania then he can retire, enter the HOF and have a lavish end to his perfect career. I don't think that making a deal with McMahon is any longer equated to a deal with the devil, I know I'd personally lose no respect for Sting if he changed his mind. I guess 21/2/11 is the deciding factor, for me if it is simply Undertaker on that day, then it never will be Sting.