-- Chris Jericho is scheduled to wrestle Undertaker at the Greatest Royal Rumble - only the third time that they would be wrestling each other in a singles match. Jericho's Fozzy tour finishes up on April 22 and then he doesn't go back on tour until mid-July, so it's possible he could be with WWE for more dates or head to New Japan during that time.
-- Rey Mysterio will be wrestling for WWE at the Greatest Royal Rumble as a one-shot deal. While he continues to talk to WWE about a more regular return, the two sides had no agreed to anything as of this writing and even if he did return to WWE part-time or full-time, it wouldn't happen right away as he has to fulfill a number of commitments for other promotions.
Source: Wrestling Observer Newsletter. Subscription information & prices available at f4wonline.com