How about the Yorkists emerging as a rebellion. The conduct of the War of the Roses would be satisfied through this because the battles of the wars were fought to sap the man power of the other army to neutralize it as a political threat. There fore if the Yorkists are rebels, then a lancastarian player's objective would be to wipe them out, which would be historically accurate. If the Yorkists started with provinces then seiges would have to be fought and land re-conquered, historically this wasn't the case. The only problem with this is, there would be no option to play as the Yorkists in the campaign game.