Key derivation enables pseudonymity by enabling users to use different keys for different interactions.
Payments should be negotiated using partial transactions with minimal extra data.
A service provides HTTP URLs for a user to access some sort of service.
A messages service provides HTTP URLs for a user to receive messages.
A paymail service provides wallets the ability to create paymail handles to receive payments.
A headers service provides wallets the ability to keep up to date with Bitcoin block headers so they can verify chain POW and tx merkle paths.