My dream would be a product offering that offered pass-through payment processing, so my vendors have an account and give my store a key. Payments go directly to the vendor, and a pre-negotiated amount or percentage is automatically collected and sent to me. Both me and the vendor pay the same processing rate.
I have several concepts I'd love to build and launch, but all of them fall down because I can't easily justify a 2x2.9% transaction fee.