Whoever they choose they need to choose wisely since they've been hyping this as a high-profile feud. Choosing a jobber like Erick Rowan would be so disappointing. Also if it turns out to be disappointing the fans might lash out on Reigns and the mystery partner and just cheer Ambrose. Lets look at the candidates so far
Randy Orton - He might be the mystery partner but in my opinion he isn't going to be. That attack was just to set up a future feud between Orton and Wyatt. They're just trying to make it more suspenseful by having the Wyatts take out multiple superstars so Reigns and Ambrose have nobody to choose. Also here is further proof from Smackdown :
Erick Rowan - Would be so disappointing. Nobody cares about him; he's nothing but a talentless jobber.
Samoa Joe - I think he would be a great fit with the 2 Shield guys. Honestly he is kind of like a mix between Ambrose and Reigns. He is Samoan and badass just like Reigns, but he doesn't have the "look" that Vince loves and he is also from the indies and does a lot of moves. Also he doesn't have anything going on right now and this would be a great way to use him.
Baron Corbin - He's way too green and I don't think he will fit in at all.
Big Show - This would be disappointing as well, its like his 100th face turn and he doesn't fit in with The Shield.
Kane - Wouldn't make sense either because he has unfinished business with Rollins and Lesnar.
Cesaro - He teamed with Reigns and Ambrose before and his feud with Owens is apparently over. This would be a good way to keep him relevant.
Seth Rollins - if Rollins is headed for a face turn they can try to fit him in here
Randy Orton - He might be the mystery partner but in my opinion he isn't going to be. That attack was just to set up a future feud between Orton and Wyatt. They're just trying to make it more suspenseful by having the Wyatts take out multiple superstars so Reigns and Ambrose have nobody to choose. Also here is further proof from Smackdown :
Jimmy Uso was also attacked and injured by the Wyatts.
Erick Rowan - Would be so disappointing. Nobody cares about him; he's nothing but a talentless jobber.
Samoa Joe - I think he would be a great fit with the 2 Shield guys. Honestly he is kind of like a mix between Ambrose and Reigns. He is Samoan and badass just like Reigns, but he doesn't have the "look" that Vince loves and he is also from the indies and does a lot of moves. Also he doesn't have anything going on right now and this would be a great way to use him.
Baron Corbin - He's way too green and I don't think he will fit in at all.
Big Show - This would be disappointing as well, its like his 100th face turn and he doesn't fit in with The Shield.
Kane - Wouldn't make sense either because he has unfinished business with Rollins and Lesnar.
Cesaro - He teamed with Reigns and Ambrose before and his feud with Owens is apparently over. This would be a good way to keep him relevant.
Seth Rollins - if Rollins is headed for a face turn they can try to fit him in here