I think using google authenticator is not enough. Many of us may just don't have a smartphone, or don't want to use google services.
Why don't you add another 2fa option to increase security level? I think using another passphrase, chosen by the single user, would be the best option.
As far as I know (didn't use it for a while) , Electrum have 2 Factor authentification using both Google Auth & TrustedCoin (it dosen't require a smartphoen as far as I know) and according to an article on CoinTelegraph it says :
The update unsurprisingly focuses on security. They have added multi-signature functionality using parallel BIP32 derivations, and 2FA support using the Google Authenticator and TrustedCoin services (wallets can be restored by seed if TrustedCoin ever shuts down). Support for popular hardware wallets Trezor and Btchip have also been added.