The only true building blocks we have right now are Rondo and Bradley. They are two established players in the league who have proven that they can play well together while having elite attributes (passing for Rondo, defense for Bradley). These are the only two guys who I can safely say will be here in the next couple years once we rebuild.
Jeff Green has the potential to be another piece. He's proven that he can be a high-level backup on a good team, and has shown flashes of excellence when inserted into the starting lineup. If he can develop any sort of consistency, he can become a solid starter in the NBA. That's probably his ceiling. If he does continue to improve, his contract might finally be tradable, and we could possibly move him in a deal for another player, preferably one who performs and shows up every night. Green's future in Boston has yet to be determined.
When Danny drafted Sully and Melo, I'm assuming that he intended those two to be assets primarily, and, if trades could not be made, our front court for the future (in 4 or 5 years). Sully has proven he has an elite skill (rebounding), but unless he can improve defensively, as well as shows signs that he can score more consistently, he's probably better off as a team's first big off the bench. Melo has shown us next to nothing in his few appearances in the NBA, and I have my doubts that he'll improve to be anything other than an average backup center. We'll see if he can improve over the next two or three years.
Courtney Lee and Brandon Bass are guys who are great off the bench for good teams because they provide skills that are very helpful. I don't expect either of these guys to be a part of our rebuild, as they're best suited as supporting pieces on winners, not teams that are rebuilding. Both are solid trade assets at best, so it might be tough to move them.
Terrence Williams, DJ White, and Jordan Crawford are really just fillers in trades to me. We've seen what Crawford's done on rebuilding teams before (in Washington), and we've read about Williams' antics in NJ (bad attitude). White seems like a great eleventh or twelfth guy on a rebuilding team, so maybe we keep him around. Personally, I wouldn't.
All that being said, here's a realistic view of our team going forward (once Pierce, KG, and Terry either retire/get traded):
*This team does not include draft picks that we make in the future, as it's [dang]-near impossible to predict those.
Rondo
Bradley
Green
Sully
Melo
That's our starting unit. Depending on how you project the development of these players determines how many games they would win. In my opinion, this starting unit, along with a mediocre bench (which most rebuilding teams have), wins you probably 35-40 games at most. Not enough floor-spacing, not big enough, and a lack of shot-creators really hurts this unit. They could be decent defensively, if only for the brilliance of Bradley picking up guys full-court.
Crawford
Lee
Williams
Bass
White
Let's say no free agents want to come to Boston (which is absolutely realistic). That leaves us with the players we already have. This bench is truly awful. The only above-average defender is Lee, the size is terrible, and I can't see this unit putting very many points up on the board. When our starting unit goes out, the other team is going to have a field day.
So there, that's our realistic future. I'm not really surprised that the future looks a little dim, as Danny has currently constructed this team so that it can win now. By doing that, you have to make sacrifices. In this case, Danny sacrificed the future.
Development could be a huge factor in whether this team is lottery-bound or early-round fodder in the playoffs. It really depends on how you view these players.
One last note: Danny Ainge has stones. That can be good and bad. We've seen him land two stars in the span of two months, rejuvenating the franchise and landing one championship. I wouldn't rule out that he could do this again. If anything, it's realistic to assume that Danny, once the rebuild hits, will be working the phones to make us a contender again, and save us from the mess of a team I showed you above. Hopefully, he pulls it off.