#4 The Undertaker

Mark Calloway, the wrestler who has portrayed the Undertaker character for more than two decades, has been in the wrestling business for a very long time under a variety of different gimmicks.
In his early days, he wore a mask and called himself Texas Red. Then he became an ex-military special forces veteran The Punisher -- no relation to the Marvel character--and again he was rebranded as Mean Mark Callous in WCW.
But when he became the Deadman, the gimmick struck gold. For whatever reason, Calloway and the Undertaker are a perfect match. It's the perfect match which means you won't be seeing Undertaker in AEW.
Calloway couldn't use the Undertaker character in other promotions because it is a trademark of WWE. Without his famous deadman gimmick, Undertaker is just an ageing part-time wrestler whose career has been semi-over for years. It's hard to imagine him having any interest in working for AEW unless it was in a backstage producer capacity.