One of the ideas about All-Stars is that it's the new generation versus the old generation.
Current stars versus the Legends.
Who or what would be "dream match" material?
Also, your guess can be blacklisted or unpopular wrestlers just as long as they worked in WWF/E at sometime or another. Even guys that Vince would never allow (Savage) or guys that will never be featured for legal reasons (Owen)
Here are some I can think of...
John Cena vs The Rock (obviously)
Big Show vs King Kong Bundy
Stone Cold Steve Austin vs Randy Orton
Daniel Bryan vs Bret Hart
Jack Swagger vs Lex Luger (Or Kurt Angle)
Mark Henry vs Bam Bam Bigelow
CM Punk vs Rob Van Dam
Vladimir Koslov vs Nikoli Volkoff
Santino Marella vs Doink The Clown (Don't laugh. You know you want to see it happen.)
and that's all the matches I can think of for now...