Locke and Gau need the most work imo.
Locke looks like a 10 yr old. Why? Ill-pronounced chin and big eyes, with big ear as well. His bandanna should be at the same slope as the rest of his head/face. His hair strands are too big and clunky. Look at Terra's hair and the hair on the orig Locke face. Smaller strands, more shaggy will make it look more natural. Get rid of those 2 vertical pixels at his nose. Pronounce his jaw/chin and make his mouth wider. Also another detail from the orig face, he has beady little eyes looking to side, insinuating that he is clever and mischievous, matching his personality in the game. I know you're putting a new style on it but if you study certain elements that worked, you can convey them in your own as well.
As for Gau, the orig face was really strange and awkward pose, hunched over, but it conveyed his animal instincts in a way. The hair also lacks flow, again the strands are too large, making it look DBZ-esque. His jaw is angular to the point that he has no chin practically. His eye-ridge is rounded to the point where it is taking over the rest of his face and thus the head volume looks wonky. To portray his characteristics more closely, again look at the subtleties of the orig. A 2 vertical pixel smirk, curved eyebrows and eyes looking off to the side. That, combined with a posture of being hunched over gives the idea that personifies his aboriginal demeanor.
I hope this helps.