Orks are pretty free form when you think about it. Really like Fire? Bam, your a Burna Boy; Go light some 'umies up. Like going fast? How would you like your speed? In jetpack, car, helicopter, or bike form? Hell, you want to beat the tar out of little things? Became a Runetherd. Orks are just based around what an Ork wants to do, and how he can best apply it to beating the crap out of other people (Which is something all Orks want to do, so hey, living the dream right?).
The Imperium's got nothing on the Greenskins