Adore Cyborg and Zatanna, but Martian Manhunter is an easy vote for me, with Cyborg a strong second.
I'd also like Harry Lennix (or maybe Doug Jones?) to play him.
Leave Zatanna to Dark Universe/Justice League Dark. She'll shine better there.
Also would consider John Stewart instead of Hal. Fresher attempt at Lantern after the last movie flubbed, fans are broadly familiar with him thanks to the cartoon, and it adds some diversity in the cast to broaden the spectrum of relatability. If we aren't going to do Cyborg, I would like John to fill that important gap. I know focusing on skin color might seem petty to some, but black folks deserve some more representation, some brighter role models. Superhero films have been severely lacking in that.
Plus I just like his personality more anyway -- though I hear Hal's personality is much more enjoyable in the recent CG animated series than typically portrayed... Still need to give that show a whirl!