I think a futuristic race would be cool (go with something unique and add two more classes to humans) Maybe a LA explosives class to give us another ranged physical class and perhaps an AA scientist class.
Thing is this, they came from the same future where Chronobikes came from, their city and everything is located through a portal.
Maybe the story could be something like the wraiths have taken over the future, wiped out untamed, tideborn, earthguard, and elves. Humans are the only ones left after the desolation. Humanity now exists in small numbers that dwindle every day, so they develop a way to come back in time to help the races fight against the wraiths.