I don't think there's anyone quite at that level YET, but one name to watch in future seasons is Jurickson Profar, a shortstop in the Rangers' organization.
If you're looking for something to help you this season, Nolan Arenado should be up by June and play third full-time for the Rockies. He led the minors in RBI last year and is off to a great start at Double-A. I'd be surprised, actually, if he doesn't make a bigger fantasy impact this year than Harper and Trout. I could also see Brett Jackson getting the call to the Cubs by June and outproducing Harper and Trout the rest of the way, largely because I think he'll play more. Jackson isn't the next BIG thing, but he's projected as a Chris B. Young type for many years to come.
On the pitching side, Trevor Bauer might be the next big thing, but he's unlikely to be a stud right away -- few pitchers are. His control is still a bit of an issue, so I think he'll be great one start, ugly the next.
Rizzo projects as a stud long term, but Theo Epstein wants him at Triple-A Iowa all year, so I wouldn't count on him before September.