With much of the plot revolving around Apokolips and Darkseid, Aquaman would get the shaft. Plus with MM, they already have someone who is his equal in Superman so I can see the audience not caring for either or the heroes struggles if they have two godlike members...and then also have Batman, Martian Manhunter, and Wonder Woman. If MM is in it, just do a cameo as a detective while he is in human form.
I'm perfectly fine with this and very happy they are only focusing on those 5. Do I think Barry and Hal shouldn't be the legacy characters they use? Yeah.
I'm actually very happy with how Justice League is moving along. Also, it makes a lot of sense why they are doing this. They reportedly are going for the exact opposite of The Avengers formula. Instead of building to a single movie with separate franchises, they are going to use JL as a jumping off point for the Batman reboot, Wonder Woman, and a Flash movie.
But I digress...Flash would steal all the spotlight so its probably best if they limit the team. Don't want him outshining everyone, just 3 or 4.
Oh and Ryan Gosling for Wally West