I don't know that Poloniex has any plans to add USD, but you should absolutely keep an eye on @bittrex. They have made it very clear that they are in the works of adding USD along with a new and very much improved UI design to the site.

@bittrex-bill and @bittrex-richie have both made statements about it and they have been in the process of taking care of all legalities to get USD for months now.