As noted, at the end of Tuesday's episode of SmackDown Live, Randy Orton confronted WWE Champion Bray Wyatt and said that he pledged his undying allegiance to him and could not face Wyatt at WrestleMania 33. According to the WWE "rule book," with Orton giving up his WrestleMania title shot, Roman Reigns should now be facing Bray Wyatt at WrestleMania 33 for the WWE Championship. There is actually precedent for this situation.
Austin informed McMahon that he would be wrestling for the WWE Championship at WrestleMania and then-WWF Commissioner Shawn Michaels appeared in the camera shot with Austin. It was clear that the two had been partaking in some adult beverages before their promo was shot. Michaels informed Vince that the official WWF rulebook stated that "if the winner of The Royal Rumble is either unable or unwilling to go to WrestleMania to fight the champion, then the runner-up gets all rights and privileges."
In this case, the runner-up of the 2017 Royal Rumble is Roman Reigns. Obviously this was a wrestling angle, and it's highly unlikely that a legitimate WWE rulebook exists (although WWE did announce yesterday the release of The Official WWE Book Of Rules (And How To Break Them)). Even if it did, it likely does not have a provision regarding a Royal Rumble winner giving up their title shot at WrestleMania. But if it does, then Reigns should be contacting a lawyer.
As for Austin, he put his WrestleMania title shot on the line for a steel cage match against Vince McMahon at St. Valentine's Day Massacre: In Your House, which was McMahon's first-ever singles match. Austin ended up winning the match and went on to defeat The Rock at WrestleMania 15 to regain the WWF Championship.