Our goal is to make the Slugga the most common Ork class on the battlefield and we are looking into ways to incentivize players to play this class. Sluggas are very strong at close/melee range, have a wider selection of pistol types to choose from than other factions, a Waaagh! mechanic that is particularly beneficial for them, etc.
Eldar have two footslogging melee classes, each with very different mechanics. The Striking Scorpions are a stealth class, and the Howling Banshees charge forward and suppress their targets with a scream to make it harder to take them out in the final approach.