#2 Demon Balor
Finn Balor is one of the most underutilised superstars on the main roster right now, but his Demon gimmick is the most protected character in the entire WWE, which includes Roman's push and Lesnar's invincibility. Vince might think that Finn isn't cut out for the Universal title, but he certainly does believe in Balor's Demon avatar.
Currently not involved in any feud, Balor is free to accompany Undertaker against DX. Demon Balor also has that big fight feel around him that can replace a big match up between DX and the Brothers of Destruction. It could be a one and done scenario just to cope up with the last minute replacement.
#1 Aleister Black
Aleister Black was out with an injury when he was scheduled for a triple threat match against Tommaso Ciampa and Johnny Gargano. He was taken off TV after a backstage attack angle. While the recent reports suggested that he could come back to the ring in October and he has already started training heavily.
Fans already believe Black to be the second coming of the Undertaker and this could be a great start to his run on the main roster. Undertaker could also take the role of his mentor going forward. An NXT call up could just be the moment that can fill the gap of Kane and Undertaker teaming up.