Thumbnail sketch: Two-factor (Yubikey or Google Authenticator) protected wallets
Hardware:
Computer. Shared-secret authenticator (Yubikey/Google Authenticator/etc). Server (possibly shared with millions of other users).
Motivating use case setup:
User creates a split (2-of-2 multisig) wallet on the computer and server. Keys must be securely backed up to protect against loss. GUI to be determined, but there will be some type of "Use Authenticator" checkbox specified at setup.