I'm still building this out; plan to add reasons around the players and probably build up the list of maintainers. But, looking forward to hearing others' thoughts
Maintainers -- Under maintainers, I've grouped the players who have mostly played up to expectations and therefore maintained their prospect status about unchanged to the beginning of the year.
Alan Webster---seems on verge of becoming a riser; has been very good lately but I want to see a bit more sustained reduction in the walks
Henry Owens---as my #2 coming into the season, it would be almost impossible for him to rise. Certainly has been great and continues to be a must check in the boxscores
Blake Swihart---see Owens. Only concern is drop in walk rate. Nice increase in power makes up for it. Has certainly positioned himself to leap over Vazquez on the catcher depth chart.
Garin Cecchini---As mentioned below, could be a bit of a faller, due to lack of power and drop in average over the past 6 weeks. That would be a harsh reaction to his first professional slump though. Wait and see for now.
Christian Vazquez --- see Cecchini. Continued Lack of power suggests more likely to be closer to a backup on a championship team than a starter
Matt Barnes---struggling a bit after early season injury. Not overmatched though. Another one to wait and see.
Bryce Brentz---Not going to drop a guy who had a great spring training just for bad start prior to an unlucky injury. The Red Sox need him to develop.
Wendell Rijo---started out extremely hot, nearly matching Mookie Betts for the first month or so of the season. Has since cooled off considerably. However, remains among the youngest players in the SAL and with very respectable OBP/SLG numbers for a teenaged middle infielder.
Manuel Margot---See Wendell Rijo. Has more than held his own in an age appropriate league. Not excelling to the extent that you'd have him rising into a top 10 prospect placement though, and given that most had him in the mid-teens last year he's not really a riser.
Corey Littrell---unexciting soft tossing college lefty drafted in first few rounds having unexciting first full season at age appropriate Salem. Struggled with control for a couple starts, but seems to have recovered command. Meh.
Risers--To be a riser in this context you have to have significantly exceeded expectations and improved your stock since the beginning of the season; just having a good season is not enough to be in this category. They are roughly ranked in order of how much they have improved their stock, not necessarily by how good a season they are having (I didn't include DLR or Workman here, as some publications don't consider them prospects, but otherwise, they'd be here).
Mookie Betts --- starting in RF before the all-star game; self explanatory.
Rafael Devers --- I am normally loathe to rate 16 year olds very highly, especially on the basis of Dominican Summer League stats, but this one seems to be really, really special. Destroying a league not known for having players put up gaudy offensive numbers, and rumored to be headed to the GCL a year ahead of schedule. Will have to be ranked in your top 10 at the end of the season, even if he doesn't get another hit all summer.
Brian Johnson --- Has been nearly dominant in AA, after pedestrian career through 2013 partially because of injury. Though most prospect raters continued to high on him, prior to this season he could have almost been characterized as a bust. Now looks like he'll be a top 10 prospect in a system stocked with high minors pitching depth.
Anthony Ranaudo---Producing a low ERA in AAA and staying healthy means he's moving up my list. I almost left him on the maintainers list because of the elevated walk rate and occasional lack of stamina. But they paid huge dollars to sign him, so I'm succumbing to confirmation bias.
Devan Marrero --- As noted below, his emergence as a legitimate offensive threat in AA has to be considered one of the feel good stories of the year. According to a couple articles I posted in the Adopt a Prospect thread, the improvement reflects conscious adjustments in his approach and mechanics rather than merely a half season hot streak. Moreover, Farrell called his defense, "Just as effective as" Iglesias. If the hitting keeps up all year, he should be a top 5 prospect this offseason.
Travis Shaw --- Showing that his rejuvenation in the Arizona Fall League was for real. Earning the promotion to AAA is a huge coup for his development, and he's continued to produce despite the higher level of competition.
Steven Wright
Carlos Asuage
Reed Gragnani
Henry Ramos
Sean Coyle
Joe Gunkel
Jantzen Witte
Savers -- This group are the players who were probably one bad season from being released or relegated to backup roles around the system at levels that needed their skills, rather than getting opportunities to improve those skills. Yet, they are vastly exceeding expectations this year, though sometimes at levels they should have mastered a year or two ago.
Kendrick Perkins
Pete Hissey
Derrick Gibson
Jonathan Roof
Carson Blair
Aneury Tavarez
Fallers -- Prospects who have significantly lagged expectations or are falling behind age-appropriate levels for real prospects.
Drake Britton---from the precipice of Boston's bullpen in mid-March to mop up in Pawtucket.
Alex Wilson---see Drake Britton, regressed badly.
Pat Light---looks like a 2nd round bust
Cody Kukuk---just can't get the walks under control.
Ty Buttrey---I remember reading a quote somewhere that "if the kid you're negotiating with is acting as if the signing bonus is the biggest check he'll ever cash, walk away". Buttrey fits that statement so far.
Trey Ball---when the 7th overall pick has an ERA of 7 and isn't showing mid-90s velocity there's a big problem.
Teddy Stankiwicz --- similar to Ball. Performance hasn't been as alarmingly bad, but very inconsistent season in low-A despite a year of highly competitive JuCo competition is disappointing to say the least.
Dan Butler---like many at Pawtucket, not hitting at all
Alex Hassan---see Butler, though showing signs of life since his return from cup of coffee.
Forest Allday---only holding his own at Greenville despite being a 4th year senior sign
Tzu Wei Lin---not developing with the bat at all
Heiker Menesses---regressing with the bat, being lapped on the middle infield depth chart
Maintainers -- Under maintainers, I've grouped the players who have mostly played up to expectations and therefore maintained their prospect status about unchanged to the beginning of the year.
Alan Webster---seems on verge of becoming a riser; has been very good lately but I want to see a bit more sustained reduction in the walks
Henry Owens---as my #2 coming into the season, it would be almost impossible for him to rise. Certainly has been great and continues to be a must check in the boxscores
Blake Swihart---see Owens. Only concern is drop in walk rate. Nice increase in power makes up for it. Has certainly positioned himself to leap over Vazquez on the catcher depth chart.
Garin Cecchini---As mentioned below, could be a bit of a faller, due to lack of power and drop in average over the past 6 weeks. That would be a harsh reaction to his first professional slump though. Wait and see for now.
Christian Vazquez --- see Cecchini. Continued Lack of power suggests more likely to be closer to a backup on a championship team than a starter
Matt Barnes---struggling a bit after early season injury. Not overmatched though. Another one to wait and see.
Bryce Brentz---Not going to drop a guy who had a great spring training just for bad start prior to an unlucky injury. The Red Sox need him to develop.
Wendell Rijo---started out extremely hot, nearly matching Mookie Betts for the first month or so of the season. Has since cooled off considerably. However, remains among the youngest players in the SAL and with very respectable OBP/SLG numbers for a teenaged middle infielder.
Manuel Margot---See Wendell Rijo. Has more than held his own in an age appropriate league. Not excelling to the extent that you'd have him rising into a top 10 prospect placement though, and given that most had him in the mid-teens last year he's not really a riser.
Corey Littrell---unexciting soft tossing college lefty drafted in first few rounds having unexciting first full season at age appropriate Salem. Struggled with control for a couple starts, but seems to have recovered command. Meh.
Risers--To be a riser in this context you have to have significantly exceeded expectations and improved your stock since the beginning of the season; just having a good season is not enough to be in this category. They are roughly ranked in order of how much they have improved their stock, not necessarily by how good a season they are having (I didn't include DLR or Workman here, as some publications don't consider them prospects, but otherwise, they'd be here).
Mookie Betts --- starting in RF before the all-star game; self explanatory.
Rafael Devers --- I am normally loathe to rate 16 year olds very highly, especially on the basis of Dominican Summer League stats, but this one seems to be really, really special. Destroying a league not known for having players put up gaudy offensive numbers, and rumored to be headed to the GCL a year ahead of schedule. Will have to be ranked in your top 10 at the end of the season, even if he doesn't get another hit all summer.
Brian Johnson --- Has been nearly dominant in AA, after pedestrian career through 2013 partially because of injury. Though most prospect raters continued to high on him, prior to this season he could have almost been characterized as a bust. Now looks like he'll be a top 10 prospect in a system stocked with high minors pitching depth.
Anthony Ranaudo---Producing a low ERA in AAA and staying healthy means he's moving up my list. I almost left him on the maintainers list because of the elevated walk rate and occasional lack of stamina. But they paid huge dollars to sign him, so I'm succumbing to confirmation bias.
Devan Marrero --- As noted below, his emergence as a legitimate offensive threat in AA has to be considered one of the feel good stories of the year. According to a couple articles I posted in the Adopt a Prospect thread, the improvement reflects conscious adjustments in his approach and mechanics rather than merely a half season hot streak. Moreover, Farrell called his defense, "Just as effective as" Iglesias. If the hitting keeps up all year, he should be a top 5 prospect this offseason.
Travis Shaw --- Showing that his rejuvenation in the Arizona Fall League was for real. Earning the promotion to AAA is a huge coup for his development, and he's continued to produce despite the higher level of competition.
Steven Wright
Carlos Asuage
Reed Gragnani
Henry Ramos
Sean Coyle
Joe Gunkel
Jantzen Witte
Savers -- This group are the players who were probably one bad season from being released or relegated to backup roles around the system at levels that needed their skills, rather than getting opportunities to improve those skills. Yet, they are vastly exceeding expectations this year, though sometimes at levels they should have mastered a year or two ago.
Kendrick Perkins
Pete Hissey
Derrick Gibson
Jonathan Roof
Carson Blair
Aneury Tavarez
Fallers -- Prospects who have significantly lagged expectations or are falling behind age-appropriate levels for real prospects.
Drake Britton---from the precipice of Boston's bullpen in mid-March to mop up in Pawtucket.
Alex Wilson---see Drake Britton, regressed badly.
Pat Light---looks like a 2nd round bust
Cody Kukuk---just can't get the walks under control.
Ty Buttrey---I remember reading a quote somewhere that "if the kid you're negotiating with is acting as if the signing bonus is the biggest check he'll ever cash, walk away". Buttrey fits that statement so far.
Trey Ball---when the 7th overall pick has an ERA of 7 and isn't showing mid-90s velocity there's a big problem.
Teddy Stankiwicz --- similar to Ball. Performance hasn't been as alarmingly bad, but very inconsistent season in low-A despite a year of highly competitive JuCo competition is disappointing to say the least.
Dan Butler---like many at Pawtucket, not hitting at all
Alex Hassan---see Butler, though showing signs of life since his return from cup of coffee.
Forest Allday---only holding his own at Greenville despite being a 4th year senior sign
Tzu Wei Lin---not developing with the bat at all
Heiker Menesses---regressing with the bat, being lapped on the middle infield depth chart