I honestly wish English had a modern "usted" equivalent, because I've felt the need for it several times when speaking to people like professors and bosses.
It trades against the problem that you'd end up with people where formality standards are unclear and you don't know which to use. I've had professors who objected to overly-formal address, made them feel old.
I'd suggest bringing back 'thou', except given current cultural context it's going to sound more formal than 'you' and not less.
that happened to me wednesday. i was with an irish client, so when we first met, i was very polite and formal in french. then the translation started and we switched to english, and it sounded downright casual compared to when we spoke french. turned out it was a blessing in disguise as the session was about some very serious family business, and the family enjoyed the casualness my translation injected into the conversation.
also, first time i met my (american) anglophone coworkers, we instinctively switched to informal in french to try and grab back some of that casual feel from english. we got some odd stares from the russophones, talking with informal construction within minutes of meeting each other. it would seem maghrebi arabic is similar in that regard.
any pretense at knowing what we're doing flies out the window when addressing or getting addressed by english anglophones, though. "tu" and "vous" become interchangeable, which is part of the anglophone charm, or so i'm told.
to address your last point, sigma, i don't think bringing back thou is necessary, all we need to do is define clearly whether a language is formal or informal at its core. if it's informal like american, then use honorifics, like "sir", "ma'am", "miss" etc... that's how i've been winging it for the past few years, and it seems to be working, although i get "aww, cute, a little yankee" looks from englishmen.
