Short version: A lot.
Umarak: Maybe change his shoulder placement. Overhauling the leg build to look better.
Onua: Changing his shoulder pads, changing torso build, placing gun on shoulder, changing hammer in a pick axe or a rocket hammer, and placing the Skull torso as a thigh cover.
Lewa: Giving him some more green, longer arms, little rotor arms on his back to put heli-blades for flight mode, and his changing custom limb build. Currently the build for his arms looks bad, and the one for his legs is just unnecessary. You could acheive the same (or better) effect by just attaching them to a regular bone.
Gali: Changing thighs, Orange Vorox armor is going to be placed like a half-skirt, more fins for aquatic look, and changing weapon to a claw-trident.
Pohatu: Fixing his color distribution and arm build, maybe add a cape or cloth, weapon changed to have boulders on both sides and have small cyrstals/rocks attached as well,and adding dagger secondary.
Tahu: More red, less gold. Completely changing his shoulders, torso, legs, you know what? I'm changing everything. Custom shins (one that uses real flame pieces instead of those cheap cyrstals, and you can say goodbye to those awkward thighs.) and Torso, changing the shoulders, changing sword to look better and mod the flame extension function, and going for the flaming Samurai look.
Kopaka: He will be scrapped almost instantly. If I keep him his shins are going to be gold and hold the cyrstal armor, his sheild is actually going to look like a sheild, and he's going to get bulked up.