What do you think about a Proof of Identity system?
In which sense?
If you mean a totally descentralized, online system of identity verification - that is very difficult, perhaps impossible. It's difficult, if not impossible to tell if an account is a real person or a bot, and if a person has many identities.
Something like Ripple is the most I can imagine, but even there, you can have multiple identities inside the system.
It may be a different thing if you allow trusted entities to participate, like goverments, organizations or even "organized, trustworthy offline groups".
