Thanks! Would love to see a file sync app, an MLS-based chat (where the encryption key is essentially a combination of the keys output from MLS and the PTK from FOKS). Password managers. I think there's the potential for something like a Hashicorp-Vault-style server-side secret key material manager, but many details left to reader. Maybe a Skiff-style Google-docs clone? I think there are lot of potential directions to go in.
IMO Vault is really nice, but something as simple as possible is better for managing secrets, especially when the storage layer has permission and sane encryption handled for you.