In the current iteration of the game greenskins have way too unified factions, while in the lore they had been incredibly fractured. My idea is, that the greenskins should only be allowed to own their capital province and provinces around it directly. Once the greeskin would expand outside that area, each province they would take spawn a new semi-independent greenskin faction. That faction could conduct diplomacy on its own, but would have to join in their overlord's wars and pay tribute. These vassal factions could not expand, each their newly captured province should become another overlord's vassal.
Oh, and also greenskin settlements should routinely spawn savage orc hordes, all which would harrass settlements of nearby races.