Eric Sachs should be Kraang
 

Since they changed it up so that Shredder answers to sachs, it would make sense in the next movie for Sachs to take his shirt off revealing himself as a robot with a giant brain living in a cavity in his stomach. Unless he dies in this movie they should go this route.

You're right.

Perhaps Eric Sachs can be an evil utrom named Kraang.

Hmmm based on what people who I've seen the movie have said

nah shredder talks in the movie in english and japanese he calls sacks his student and his name actually is spelled sacks in the movie.....
they also mention that shredder took care of sacks sometime back in japan...

shredder and the foot funded the project sacks and aprils dad were working on with splinter and the turtles...

and sometime later sacks is still working under shredder and they find out the turtles are still alive they put there plan back on track

edit: i really wish that utroms were in it, or atleast krang but they werent :(

I read in another post that Shredder was Sachs muscle, but if he is indeed a student of the shredder it would make less sense. I hope we get to see a more realistic and more ruthless version of the '87 Krang in the next movie. If they throw in Bebop and Rocksteady there's no way it can't make money