Building a team doesn't just happen through the draft. Savvy teams can find gems through free agency or even the trading process in order to keep the system they have and maximize value. There are a number of players in the NFL who could transition to a role similar to Harvin's. Some teams have already started, thanks to Harvin's success in Minnesota.
The first name that comes to mind is also the player who might be most familiar of the list to Vikings fans: Randall Cobb.
It is undeniable that the Packers have been grooming Cobb to fulfill a similar role to Percy Harvin, and they've had him run reverses, end arounds, sweeps, dives and anything else you might want from a speedy running back.
They've also had Cobb returning kicks and punts, to great effect.
Naturally, Cobb has been an effective receiver as well. He does a good job generating yards after the catch, although his capability isn't nearly the same. His agility and athleticism are nearly on par with Harvin's, but his ability to create separation with that athleticism is a little behind.
He could do more to improve his route running and footwork, and be a more reliable target for an all-world quarterback like Aaron Rodgers. He also doesn't have the soft hands and haul-in capability Harvin displays, with a very high drop rate (12.09 percent according to Pro Football Focus).
Nevertheless, he's explosive and can run out of any formation (they are usually draw runs out of shotgun, however) while providing an excellent receiving option. He has excellent vision as a returner and is the closest the league has to a Harvin analogue besides the troubled playmaker himself.
The Vikings would not be able to target him for a trade, however. Besides the fact that Cobb is a growing part of an offense that expects to see the Super Bowl and plays two games a year against the Vikings, he's extremely cheap to have on the roster.
The next-best bet is probably Darren Sproles, who functionally serves that role for the Saints. While a frustrating fantasy pickup in 2012, Sproles ended the year with 911 yards from scrimmage. Interestingly, he had more receptions this last year than rushing attempts and had a nearly balanced workload the year before with 87 attempts and 86 receptions.
Sproles may have been the "first" modern gadget player, and has been extraordinarily productive catching, running and returning. In 2011, he set a record for single-season all-purpose yards. Sproles has the type of agility that can break games. His burst is elite as he explodes through seams and holes. He might be the most sudden player in the NFL.
His best talent might be the ability to execute a decision at the very last moment—no player in the NFL might be better at cutting at the last second without betraying his intentions. He runs receiver routes and displays a variety of receiver skills, including sinking hips at the breaks, exploding out of cuts and maintaining precise footwork.
Naturally his size limits his opportunities in the receiving game, and he would line up outside significantly less than a player like Harvin has, and he still needs to do more to beat press coverage.
Despite all his physical talent, his technical skills are still significantly behind Harvin's. Nevertheless, if properly utilized, he could also become one of the most dangerous players in the NFL, like he has been in the past.
For that reason, it would likely cost far too much for the Vikings to acquire him. The Saints are also competitors and may be particularly driven to go all-in on another Super Bowl run after a disappointing 2012. While they have some of the best running back depth in the league, it would be hard to pry Sproles from the team.
Andrew Hawkins might be the NFL player most likely to move (of these three) if a concerted effort was made to capture him. Hawkins may be one of the nimblest player in the NFL, despite the fact that he hasn't produced at an elite level yet.
Hawkins' greatest asset—the ability to move around the field like a ghost—shows up on returns, something he has yet to do at the NFL level. In Toledo and in the CFL, however, he was absolutely dominating. He was the fifth returner for the Alouettes, but exhibited vision to set up his blocks and make gains. He didn't take any to the house, but was extremely consistent, providing more yardage than a typical return while not buffing his stats with a long gain.
More than that, his short-area quickness has given him a decent reputation for generating yards after the catch. His hands are more reliable than Cobb's, but he is much worse in traffic. He also can't create real estate for the ball and will find himself contesting a lot more balls than he should.
That and his make-you-miss ability isn't complemented with something that Harvin, Cobb and Sproles all have: the ability to take a hit. Hawkins goes down far easier due to contact. He's significantly weaker than Harvin and Cobb and needs to spend time in the weight room before he takes this sort of role on.
He hasn't really lined up in the backfield with the intention to run through the tackles, and his strength and size are good reasons why. These are fixable, however, and he could be a poor man's Harvin if developed correctly. He further needs to develop a better read of defenses and improve his route-running, but he is an incredible athlete worth looking into.
Given that the Bengals seem interested in his development it is unlikely they let him go, but a trade for him would be significantly cheaper than for other slotback options. He will be a restricted free agent after the 2013 season, so the Bengals might have a floor on his trade value that would make it difficult for the Vikings to put together an attractive package. Still, it's worth a look.