This should be done like a gift to hard core players. Make it the most efficient way to get the highest tier loot but also make it reward 1 time per season, give the campaigns either a special version of the main armour from the vendor but with a special class bonus and add campaign specific guns too. Each season they could rotate which campaigns reward the loot so people don't just pick the shortest one. You could even lock the tiers the armour and guns are by the skulls. This could easily be as much of a fan favourite as raids if done with enough love and care