From the KSI's perspective, who else would it look like?

 

He originally looked like optimus, but ended up having an alternative appearance that is something in-between megs and prime. Looking like neither but taking distorted elements of both to create a new appearance. As i am sure megs originally intended to manipulate the design of his new body until it looked just like the old one, that length of time didn't present itself so he ended up the way he is

 

Galvatron's head is based loosely on Megatron's head which is good because Megatron has used the same head in the first 3 movies