The Teenage Mutant Ninja Turtles reboot done a lot better than some people expected, it made a fair amount of money against its budget resulting in it being given a sequel to come out in 2016. I personally have not seen the film as of yet, due to it not releasing until later this week here (UK) a whole two months later than the US market which I find annoying to say the least.
However, I do know that two of my all time favourite characters from anything did not make an appearance allowing them to come into it later, now we have some concept art for said characters including Krang. Here is the concept art for Rocksteady, Bebop and Krang.
Rocksteady and Bebop have a realistic vibe to their look, making them look this way would have been a great addition to the film given how dim and animalistic they can be at times. Krang looks like he would really suit this kind of humanised world that they are creating for the TMNT franchise, I would happily see these three conceptual ideas on the big screen, maybe in the 2016 sequel? Who knows.