Grail is just Grayven in a new packaging. "Oh look Darkseid had another kid not connected to the lore whatsoever and look how tough they are they beat up so and so".
While of these options, Martian Manhunter is the least bad, some issues I can foresee Snyder having:
A) There is no definitive expanded origin beyond "Erdel teleported him to Earth". If there's no coherent story to follow, prepare for a mess.
B) There's not much definitive Martian architecture and design to follow so I wouldn't count on Snyder or Terrio or whoever to read up on the lore about the pyramids.
C) It's an alien culture with not a lot of human faces. If the character design is anything to go by, it would be mostly dark CGI.
And D) whoever's making it... this is simply not movie material. Martian Manhunter's origin can just be a throwaway line "ooh, I was telelported here in the fifties" or something.