I'm not sure how you could actually build 2FA into the blockchain itself or if you'd really even want to try. It's more of a middleware problem.

I would be more for ledger integration with steem middleware, where I could pull my owner key into my ledger wallet and then integrate that with steem keychain or steemconnect. EOS already has the ability to manage multiple keys via the ledger. Hell, the new ledger even uses encrypted bluetooth which you could use to support phone apps like eSteem.