I wouldn't go that far. I mean you need to have some sort of 'cheese' factor to capture the original spirit/political point. I think going with a little size mismatch in casting could add to that. Urban is a lot like Tom Hardy, he has damn near magical size changing powers onscreen.
I like Mahershala Ali as an actor, but I don't think I have ever seen him pull off decent action. Whether that is on him, or he just hasn't been with a decent director for it I don't know. He does great 'drama' action, with the throw the punch and then minor beatdown, and he can shoot a gun, but actual 'fisticuffs' I just haven't seen.
Chiwetel Ejiofor is actually a really good pick. Though he lacks in the 'exudes malice' department. He can be intimidating in manner, but isn't "scary" like I would expect/want from a black trenchcoat-esque movie.
Really, I am drawing a blank for the villain. Only reason I even tossed out Colter is because Defenders has put him on the top of my "big, badass, black dude" list. I mean there is no real reason why the bad dude would even have to be black, other than that is what they did with the original. And I already added the qualifier of making rather significant changes to it anyway so the sky is really the limit there. I just really want to see Karl Urban in that type of role again. He does it so well.