Jason Heyward is a stud: he is one of the premier players in all of baseball. That said, his contributions to your fantasy baseball team might be overstated. In terms of traditional roto categories he isn't elite yet. Last year he shook off his miserable 2011 season and showed why he was once the next Ken Griffey Jr. His homerun output lept forward and he surpassed his previous best in stolen bases. Despite this progress, however, he saw his plate discipline slip and his contact ability fall. Since his rookie season, his swinging strike rate (SwStr%) has risen from 8.0%, to 9.9%, to 11.2%, where the average is roughly 8.5%. His outside swing rate has risen from 24.2%, to 28.8%, to 32.7%, where the average is roughly 31%. Neither of these rates is terrible, but they are certainly trending in the wrong direction. Despite the last two seasons, Heyward is still incredibly young - he turned 23 in August - and should improve in these areas, but he may be a relatively high strikeout hitter for at least the next few seasons.
Heyward finished with 27 homeruns last year, easily an improvement over the last few seasons and a new career best. Going into this last season, one of the chief concerns regarding Heyward was the high ground-ball rates he had posted over his first two years in the league. Last year he brought that number down from 53.9% in his sophomore campaign to 44.0%. With this his line-drive rate and fly-ball rates both reached new levels for him, at 19.3% and 36.7% respectively. The line-drive rate is close to the average, 20.9%, but still not quite there. The fly-ball rate is at last higher than league average, and indicates improving power, as the number has risen every year. His HR/FB rate, 16.9%, is well-above league average but is in line with his career rate of 16.0%. The rate itself isn't necessarily reason to be dubious of a repeat, but there are other indicators that are.
According to the ESPN Home Run Tracker, Heyward hit four "lucky" homeruns, or balls that wouldn't have cleared the fence under different conditions. This number tied him for the most in the National League. The average distance of a Heyward homerun last year was 401.6 feet, slightly above league average but unspectacular for a burgeoning power hitter. Before the 2012 season, Eno Sarris at Fangraphs did a study that showed a definitive peak for power, probably earlier than expected, around age 24. The study also looks at other aging trends and is definitely worth looking at. Heyward should probably be in the same territory next season, though improvement based on age wouldn't be unexpected as he comes into his own, especially if he can return to the approach he took to the plate two years ago.
Heyward stole 21 bases last year, a valuable contribution to any fantasy baseball team. Early projections for next season have him essentially repeating that output, but that seems optimistic to me. Heyward is a remarkable athlete, but he is 6'5" and 240 pounds. With that body it is hard to imagine any player maintaing high stolen base totals. Prior to last season, his career high came in 2008, at A ball, when he stole 15 bases. His speed score last year was 6.2, on par with players like Elvis Andrus and B.J. Upton. This number is outstanding, but it was also nearly a point higher than his career average, which is right around the league average. It might be unreasonable to expect Heyward to match last year's total, especially with a slightly below average success rate (72.4%).
If Heyward can improve his plate discipline, his batting average should improve. His homeruns will likely plateau, or perhaps increase slightly, and his steals total should regress. For next season, Heyward should probably go in the fourth or fifth round, with an expectation of something to the tune of .275/.360/.480 with 25 homeruns and 15 steals. There is room for upside here, but he probably shouldn't be going too far ahead of similar talents like Bryce Harper and Yoenis Cespedes.