If I had some free time, I would build a service that facilitates atomic swaps of ecash from one mint to another. Something like a broker, let's call him Charlie.

- Users Alice use mint A, Bob uses mint B

- Bob wants to pay Alice but doesn't have ecash from mint A

- Bob discovers that Charlie holds a balance on mint A and mint B

- Charlie offers to swap from B to A for a 0.5% fee

- Bob agrees, atomically swaps coins from B for coins form A

- Bob sends Alice coins from A

Charlie obviously announces himself, the mints he supports, and his contractual conditions, on Nostr. That's also how Bob discovers him.