#2 Kane
If not for The Undertaker, perhaps his brother, Kane, could face The Fiend at Super ShowDown. This is a match that many fans have speculated will happen at Super ShowDown later this month.
The Big Red Machine has been in a mini-feud with The Fiend in the last few months, which began last year on RAW.
Kane made an appearance on SmackDown last month, and he was interrupted by The Fiend, who tried to send a message, of sorts. Daniel Bryan came out of nowhere and attacked The Fiend on that show, just a week before Royal Rumble.
A match between Kane and The Fiend already has an interesting storyline, and it almost like a passing of the baton moment if and when the two demonic characters meet. A matchup between the two could also help keep the Bryan-Fiend storyline going, where Kane could perhaps come to the aid of his former tag team partner.
The Fiend and Kane haven't faced each other in the ring, but Bray Wyatt and Kane have, in the past.