Castillo has upside? I doubt it--he'll be 29 in July and he has shown he is not an MLB regular. A sub .300 OBP, poor baserunning instincts, and high K rate.. is it a wonder why Boston signed Chris Young? They likely see either he or Bradley(or both) being busts as regulars, especially after both of them struggled badly in Sept/Oct. And BTW, Castillo isn't hitting right-handers.. high risk he turns into an expensive platoon with Chris Young and/or Brock Holt.
As for Bradley, outside of a strong August, his numbers were terrible, and he has shown no sign of being able to hit MLB pitching.
Only Betts and Bogaerts have the best chance of improving. As for Swihart, that will depend on his defense improving.. he's been able to hit, but he was near the top in passed balls, and his defensive metrics were not that good. And of course, HanRam and Sandoval will continue to be an anchor weighing down the Sox... there's also a bigger risk that Ortiz declines in his final season, which would put Boston in serious trouble, as he was the only big power bat in their lineup last season (not convinced with Travis Shaw, especially considering he's blocked by HanRam.)
And the bullpen needs major improvement.. it wasn't just the bad starting pitching hurting them most of the season, it was the bullpen costing them games. Kimbrel is not enough.