Let's say a 3% chance when finishing any masterpiece you spawn a golden masterpiece. Can only have one and gives troops unique buffs. Doesn't count in your statue limit or people would feel pigeon holed into playing just one loadout.
Examples could be.
Heaves take 33% less damage from normal cannons.
Tanks shocked duration is 50% less.
Rifleman take 25% less damage from mines.
Grenadier no longer damages friendly troops. Or reduce the damage.
Medics turn into engineers. They won't heal each other but heal tanks for 15% more.
Im sure the forums could cook up really good buffs that are balanced these are just examples.
I know offense just got a update. Ice still feels really strong now with 3 prototypes maybe you could use golden statues to tweak and create new strategies.
Maybe it's own storage unit that unlike normal statue storage you can swap used ones in and out. Maybe a 24 hour cool down that can be sped up so supercell can make a buck and keep the game going.