Going by existing mechanics, orks would be quite easy - tribes are separate till big bosh smashes enough skulls, then you get Waaaagh! which is pretty much a tribal confederation. It breaks up after the first major defeat or when the big boss dies.
I'm not up on all the WFB lore, but used to play a fair bit of WFRP back in the older editions. So I'm not totally up on all the lore, but this seems to be the thing. Orks dont hold ground either. You don't invade their provinces and start constructing cannon foundries with local labor. You dont build a brothel to recruit rakes once you own orktown, or whatever.
Ork tribes and beastmen exist in the gaps. in the woods just away from the roads, outside of the flickering light of civilization. The empire is strong, but it holds on tenuously and the thin connections between cities are only guarded by the roadwardens. Every town -- indeed, every FARM -- is a fortress. Just outside the walls, just off the road, just past the banks of every waterway... there lurk the orks and goblins and beastmen.
So I get it. I get how you'd play the empire with random events... or even how you't play Kieslev or Brittania or a Border Prince or even the Dwarfs and Undeads... but I don't really see how you'd make the Gobbos or Orks or Chaos or Skaven to be playable factions.
I can imagine some ways, but it would be a pretty big departure from the standard mechanics of the Total War franchise.
Which is why I am so curious to see what they do.