I think it's out of Ortiz rematch, Lucas Browne (if he beats Whyte), or Kubrat Pulev. I honestly don't think he will want to face Martin, Breazeale or Takam after they have already been soundly beaten by Joshua. Wilder is out to make a statement and define himself so he can define his pay cut vs Joshua, he won't get that fighting guys Joshua has already beat and looking like an understudy. Ortiz rematch will bring in good money, everyone knows Ortiz in the US now and respect him as a very good fighter. Lucas Browne can be marketed very well as a 'great white KO hope' in the US as he's another big hitting finisher with poorly boxing skills, he'll get extra interest from different demographics. Wilder will be appreciative towards Browne aswell if he gets Whyte out of the way, simply because Hearn will easily win purse bid in a mandatory situation and force the fight to happen in UK, and Pulev is a good name heavyweight with soft hands who is very beatable who has been fishing for a good payday fight lately. If Browne wins, Browne. If he loses, whoever has the best profit margins risk/reward out of Ortiz/Pulev. In the WBC rankings, 7 guys in the top 15 have already been beaten by Joshua/Wilder, 4 won't generate enough interest or money and 1 simply isn't going to happen. That leaves Browne, Pulev, Ortiz (rematch), or Povetkin, and Povetkin can make more money fighting fringe contenders in Moscow than team Wilder could offer him, so you could probably rule him out as well.
I have a feeling it will be Brezeale, they have history, there is ill feeling and Brezeale is a soft touch. He won't fight Povetkin. All Wilder has got available is Joshua's left over, so it's either Brezeale, Charles Martin or maybe even Takam because Joshua had a long night with him, so did Parker so Wilder might want to do a faster job.
Problem solved if that stiff ***** fights Wilder, but he won't because he's a ****en scared suspect PED abusing *****! And his "what are you on about" promoter is scared also.