Heavy assault is some sort of orc analogue, big, dumb, effective. He has a large bomb strapped to his back to take out vehicles. Bomb can be shot, especially by infiltrators a he'll explode violently. He throws rocks at people, and also has a big club with a big rock at the end.

Engineer is pretty much a veterinarian that heals the animals by pokin them with big creepy needles. Has the mad scientist vibe. He has a backpack that has all sorts of shit in it, and that resupplies ammo to all the classes. He has a slingshot that fires various small, mutated animals at people that chew them to death.

Medic is some creepy elf dude that just giggles all the fucking time and shoots green healing magic at people. He is always very happy and carries various drug paraphernalia. He carries a bunch of grubs that you throw onto dead people and they slither down their throats and they come back to life. Shoot lightning bolts or something, or maybe javelins. Throwing stars?

Infiltrator is like a creepy little goblin dude, he would make weird gurgling noises when using his invisibility cloak, like he enjoys it way too much or something. He can throw some little beetles or something that seek out enemies and make shrill little noises and latch onto them. He also has a big fukkin knife, and a bunch of smaller throwing knives. He throws them really fast, like 3 a second.

Liberator are dragons of course, no arguments. They only have a set amount of fire, so you eventually have to crash them into something and they explode.

ESFs are the giant eagles from lotr that you can use to swoop down and pick allies/enemies around, getting killed by one would be very fun, you would just fall a fall a thousand feet and go splat, maybe try to land on someone and crush them. You could carry an ally or two around in your claws. Fighting another eagle would just be crashing into one another, and one of you eventually dies and explodes.

Galaxies are giant flying beetles that carry like 20 goddamn people. No instant action, only beetle transportation. They have little turrets that you can shoot streams of noxious liquids at things.

Flashes are just giant fucking cheetahs that you ride around at like 200 mph. They are just stupid fast, nothing can catch up with them and you just crash into shit regularly and explode, good at taking out tanks when they are at max speed.