|An original idea for Kaytooso|
He was arguably the best thing about the movie. He was witty, dark and as noble and heroic as his organic counter parts and like in the case of C3PO and R2D2 so many times, the human heroes would have failed if the tin cans hadn't done their part.
Doug Chang has the real intel via an article by Bryan Young on Star Wars.com.
"K2-SO was going to have a transparent head. At one point, Edwards toyed with the idea of K2-SO of having a transparent head so the audience could see him thinking.
Chiang explained why this didn’t quite work. “When we drew this, it looked like we pushed it just a little bit too much out of the Star Wars box.”
When that didn't quite work, they tried to but the head of a RA-7 droid on the robot. It was apparently a favoured look for a while but it didn't stick.
If you're wondering what an RA-7 droid looks like, think back to the Sandcrawler in ANH:
Young writes "late in the piece a member of the creature department, Luke Fisher, painted the distilled essence of what Kaytoo would be in a painting, which would be recognizable as the lovably sarcastic droid in the film."
You may have seen this below Rogue One concept art that came out at the 2016 Celebration Event - you can see an early incarnation of K-2SO on the far left.
In the above Rogue One squad design, you could be forgiven for wondering who that big green guy with the furry squirrel like thing on his back is.
They were known as Senna, “a Chewie type,” and Lunka, “something that could skitter into an air duct, almost like a little thief.” The characters were dropped and one wonders if it was a bit close to Galaxy of the Guardians Groot and Rocket in concept...
As an aside, check out the influence Star Wars had on Guardians of the Galaxy.
This cool set of designs in from The Art of Rogue One: