Unique reward, unique playable character (early psi op, alien, MEC, Van Doorn with unique perk combos), also rare or one off ingredients to manufacture single weapon upgrades or weapons/armour. 3 mission arcs, maybe missions that include a strategic choice - get this reward, but lose this region/reduce supplies, trigger dark event. Make use of low use maps (avenger defence, UFO raid). Generally hell yeah, l would be very interested in anything that interrupts repetition, particularly as people seem to be hankering for longer/more missions campaigns.