Take it with a grain of salt as always. From CinemaBlend
I personally think he'd kill it as J'onn. And no, not just because he already looks like an alien. There's a couple villains he'd be good for too, but none I can see being in BvS or JLA. Except maybe Brainiac.
When it comes to a rumor of a celebrity's involvement in a particular project, there's typically one specific role they're supposedly earmarked for. In the case of Benedict Cumberbatch and DC, there's not only one but two different roles he's supposedly being considered for in both Batman v. Superman: Dawn Of Justice and Justice League. One role, that of Martian Manhunter, didn't surprise Cumberbatch when he was asked about it at last night's GQ Man Of The Year awards. However, there was one role that even he didn't know he was being considered for.
BBC Newsbeat reports that in addition being considered for the green skinned martian bountyhunter/future member of the Justice League, he is also being considered for a villainous role in Batman v. Superman: Dawn Of Justice as well. If Cumberbatch's comments are any indication as to the degree of truth to these rumors, one rumor sticks to the wall better than the other, as he responded, "That's a fun one, I've not heard that one until tonight," in regards to the Batman v. Superman: Dawn Of Justice role.
I personally think he'd kill it as J'onn. And no, not just because he already looks like an alien. There's a couple villains he'd be good for too, but none I can see being in BvS or JLA. Except maybe Brainiac.