A tipster spotted a sign on the 14th Street side of DCUSA, next to Payless Shoes, saying that Lime Fresh Mexican Grill is now hiring. From their website, they're a Florida-based fast casual Mexican place. The menu looks pretty good, tacos, "dang quesadillas" a la Napoleon Dynamite, fajitas, burritos and the like. Like Chipotle across the street, you can order online.
They seem to get pretty decent reviews on Yelp -- 4 stars on 81 reviews for one in Miami and 4 stars for a new Clarendon location too. Their Twitter person said there is no opening date yet, but the wheels are in motion. They confirmed they will have alcohol too -- beers, frozen margaritas, and frozen sangria. Yelpers like the frozen sangria.
You can see the location above -- this is one of the last spaces in DCUSA that is unfilled, it's number 104 on the floor plan below (click for a bigger version). From my memory, only 117 next to the Vitamin Shoppe and the big spot on Irving where the Grocery Store That Shall Not Be Named was supposed to go are still left. There are also a couple of small spots in front of Marshall's on the first floor, and one on the second floor, but I don't really see them being filled anytime soon.