I think thou is second person ("you"), not third ("he" and "she"). We are in need of a second person plural though--at least outside of the South where they have "y'all" and NJ where they have "youse guys".
"Y'all" is plural, "all y'all" is more plural.
For instance, suppose you know a family of people. Two people out of that family are currently visiting you. You might say "Y'all should bring the whole family over next time; all y'all are welcome anytime." The first "y'all" refers to the two people visiting; the "all y'all" refers to the whole family.
What about "Vou" from the French "Vous" ?