They could have had Anna Kendrick, Jane Levy or Mary Elizabeth Winstead.

I can see them all as April, sure. I think Anna Kendrick might play well against Will Arnett since they both have comedic chops.

Megan Fox's acting actually got better in the sequel. Maybe she heard all the reviews making fun of her and worked on her acting more.

But I think the studio may have wanted a bigger name. Because if you asked someone if they saw the new TMNT reboot films, they'd go "Oh yeah, who's in it?" And it would be difficult to sell the movie to them if you just went "Will Arnett for one...um...Whoopi in a short scene as another...Jane Levy. Remember her from Evil Dead?"

They could've at least dyed her hair red.

I heard somewhere on reddit someone suggested Olivia Thirlby. Would have been brilliant imo. We just need to see these actresses donning the orange afro of April lol.