I'm guessing Vampire Weekend is a pretty safe bet. Fits right into their tour. I think that's a done deal. Chvrches too.
Judging from other tours/festivals happening around the same time, I'd say it's possible for the Pixies or the Replacements to be on the bill. Maybe Iggy and the Stooges, The Hold Steady, Passion Pit, the Walkmen.
As far as other bands to fill in the day, there are a ton on tour but they're all booked for other gigs. So no Yeah Yeah Yeahs, no Weezer, no Rancid, no Jane's Addiction, no Blink 182. I'm bummed that QOTSA and the Arctic Monkeys are definitely out. At this rate, I doubt DC even gets a date from Queens this year. They're pretty focused on touring Europe and other cities they either haven't done or been to in a long time. Though they haven't been here in 8 years, so I guess we'd qualify.