"There is a co-op online mode we will reveal soon," said Insomniac on May 10, replying to a fan question about Sunset Overdrive's replay value. The studio added that it's planning "lots of updates" — presumably after the game launches — and that there's "tons of stuff to find / unlock" in the game.
Our recent hands-on preview of Sunset Overdrive focused on its single-player campaign, where a new energy drink called OverCharge Delirium XT turns its drinkers into mutants. It's unclear if the campaign will be playable in co-op, but it seems like that won't be the case, since Insomniac referred to the co-op component as an online mode.
For more on Sunset Overdrive, which is set for release this fall exclusively on Xbox One, check out our report on the lessons Insomniac learned from the lukewarm reception to its most recent title, Fuse.