Backstory: I have one client who is "old school" and likes to send physical checks. I get paid slightly later but there are no transfer fees from online payments. I don't really enjoy dealing with hassle of physical checks.
The Idea: Cashing physical checks as a service.
Deep Dive: You would offer customers a P.O box to have clients mail checks. Once the checks are received, you would deposit them into your customers bank account. You would charge a fee but as long as it's less than online fees, your customers would be thrilled.
Validation: I don't think anyone enjoys driving to the bank and cashing physical checks.
Monetization: You would charge a per check fee or a small per amount fee. Invoicing software like WaveApps charges 1% for ACH and 2.9% + $0.30 for credit cards.
Steps to Launch: Find a small to medium agency that receives physical checks, offer to cash them for a small fixed fee. This would get you a first customer and allow you to sort out the logistics. As you get feedback you can scale it and explore other payment structures.