suggestions: others have talked about animosity. The best way I can think to implement this is to just give every greenskin unit except black orcs the "may charge without orders" trait. I suspect anything more complicated might be difficult.

Regarding income, their tech tree should definitely not include markets or anything. They should derive most of their income from looting, with traits and such that enhance that. I would suggest high growth rates, but also high squalor and unrest. I'm not sure if there is any way to affect their squalor and unrest specifically, so that it doesn't affect other races that gain control of greenskin territory, but they should at least have very few buildings and such that counter it. Maybe their happiness should be wrapped up their traits, which would encourage the orc player to garrison their cities with rulers. Instead of management and income, their rulers would have things like "I's da BOSS!" which would add law and happiness. Maybe something like "Get to Work!" which would add farming.

To balance the low income and happiness, they should have the option to WAAAAAGH! which would just be a crusade mechanic. Finally, they should go into horde mode, and they are probably the only faction that should do this, unless you bring in ogre kingdoms.