Michael Shannon in an interview with the Daily Beast debunked the idea of him having flippers:
"Somebody was asking me about this in New York and I made up some ridiculous bullshit answer about how my character Zod has flippers, and this went viral on the internet or something and people have been asking me about that. That is so obviously a complete bulls--t story, right? But people took it seriously."
But the much more interesting news comes after when he continued to joke about having flippers:
"Maybe they should add it, I don't know! Maybe in Part 3 Doomsday will have flipper hands. I mean, look, nothing would make me happier than to work with [director] Zack [Snyder] again. He's hysterical and he's a lot of fun to work with."
Spot the key word there? The rumour of Doomsday being a common threat that unities Batman and Superman has been around for years. And now is it confirmed? Whatever the case, it won't be Michael Shannon who will be playing him...in fact Michael Shannon isn't playing anyone.
"I wasn't there. It's a really awkward situation: I'm in the trailer for a movie that I didn't work on."
So we will either get someone else doing the face of Doomsday or just have Doomsday be completely computer-generated imagery. But then he said he wasn't working in the movie, but the Zod from the trailer looked like Michael Shannon a lot...and I doubt Zack Snyder would waste money trying to replicate Michael Shannon's face. He went on to say he did voice work, because he will be a ghost like Jor-El, but Russel Crow was on the set of Man of Steel. Also when he told the flipper story, he made a whole detailed story.
"I was in my costume, and I couldn't use my fingers because in the sequel I have flippers instead of hands, so I had these wax flippers on my fingers, and I couldn't open the door, and I could hear Zack being like, 'Where’s Shannon? Where the fuck is he?' and the whole crew was standing around. I was like, knocking on the door with my flippers, 'Let me out of here!'”
What I am trying to say is that Shannon's tongue slipped and now Warner Bros. want him to take it back. Either that or Shannon likes to talk bullshit. So the rumour of Zod's body being used to create Doomsday seems to be true. Doomsday could not only unite Batman and Superman, but also convince the heroes that there are some threats they cannot stop on their own, thus the creation of the Justice League.