I think Barnes and OG are the two guys who could still take their game to another level since their young and have shown flashes of their potential. Regardless, I think it will be tough to keep both players if the Raps swing a deal for Durant, though you never know. Though I'm sure the Nets will want at least one of Barnes or OG leading the deal.
Siakam and FVV are still in their prime, though doubt they have any more potential growth. They are what they are now at this point. Siakam is a 22 PPG, 8 REB type guy. Not sure he'll ever take the next step to be a 25-28 PPG guy. Plus trading Siakam would free up some money. Durant easily replaces him. FVV is a heart and soul guy and trading him would create a big hole at PG. If FVV stays healthy and settles into the player what he has done the past three seasons, I'll take that in a heartbeat.
Trent Jr is just a shooter and offensive minded player. If he grows into a 20+ PPG player which is definitely possible, great! Though his other stats like REB and AST will be on the empty side. Still a good 4th or 5th player to have in your rotation who can get hot at anytime and put up some points. Though if the Raps traded him, wouldn't lose sleep over it. Guys like him are easy to find and acquire. Would prefer to keep FVV and Siakam over Trent Jr if I had the choice.
Boucher I think could take the next step offensively but he won't be a significant guy. Would love to see him be a 15 PPG, 10 REB kind of guy.