Off the top of my head, there's AJ Styles, Samoa Joe, Daniel Bryan, Kurt Angle, Mickie James and Goldberg that all started in 1998 - 1999 (Goldberg was in 97 though). I think John Cena started in 1999 as well even though he wasn't popular until the 2000's, and I know Batista did because he said he tried out for the WCW Power Plant but they didn't take him, so he went to the WWE (which was still the WWF at that time). Any others ?