“Justice League” previsualization artist Greg Ecklund posted the following footage on his Vimeo account, showing concept footage created for the movie. (“Justice League” footage starts at 0:46)
gecklund_TMNT2_JL from greg ecklund on Vimeo.
Designed as a tool to help visualize complex scenes in a movie before filming, the previs scenes include Superman confronting the other superheroes upon his return from the dead, Steppenwolf’s showdown with the Amazons when he first returns for their Mother Box, and Parademons attacking the Batmobile.
I didn’t find the CGI surrounding Steppenwolf to be bad at all. I thought it looked pretty good!