If you built the coupon system you describe - one where any retailer can redeem the coupon at face value without a preexisting trust relationship with the coupon’s backer - you would not have ‘solved coupon fraud’, you would have created a cash alternative.

And a cash alternative has a bunch of problems of its own - most notably, money laundering and its utility in conducting fraud and getting payouts for extortion rackets like ransom ware.

These are issues that centralized coupons have. Trustless ones would have it even worse.

Are you sure the retail industry, or consumers, are crying out for a trustless coupon solution?