I think both Mick Foley and The Undertaker were under utilized at WrestleMania 15.
I would have actually had Mankind vs Vince McMahon, since Vince had consistently screwed with Mick Foley for about a year now at this point, including the Survivor Series 98 "screwjob" and getting the title off of him on RAW leading into WM15. This would have been a fun match for the fans to finally see Vince get what was coming to him, and keep Mankind in a top match.
And I'd have The Undertaker vs "Big Show" Paul Wight, since Taker was screwing with Vince and the Corporation at this point, Vince could have his newly debuted giant go up against the evil dead man as protection. Makes more sense than The Bossman.
However, I would not have Mankind vs Austin at WM15. The Rock was the right guy and needed to be in the main event. Maybe I'd have this match after Mania, perhaps at Over The Edge 99. A one on one face vs face match would have been interesting. Austin would have gone over though.