Nmail looks interesting (kind 1301, nostrmail.org).
Also, cashu looks interesting, but I don't know much about it (or zaps in general, tbh, never even setup an LN wallet for Nostr).
Also, I'd wonder if NIP-99 (marketplaces) might be a good fit too. I'd bet there's an overlap between whimsy-maxers on Ditto and an interest in an etsy style decentralized stor front.