Put Shadow Keys in chests at the end of arena or danger rooms. The higher the tiers, the better the chance. This could create incentive for people to explore T3 Clockworks other than FSC. If you don't get Shadow Keys, it's no big deal. The money you farmed can go towards investing into equipment for Shadow Lairs.
Bring Shadow Keys back to Boost on special occasion, like Christmas.
This belongs in Suggestions.