The Boston Celtics roster right now reminds me of one of those tile puzzles (except a few key pieces are missing). A lot of the pieces that we need are in place, but they just aren't in the right spots. Brad Stevens was still able to make it work, but it wasn't a finished product by any stretch. Adding those missing pieces would allow Stevens to shift guys over into their better places and everything would just fall into place much better than it did at the end of the season.
A lot of people are asking the question "who will be back next year?" or even "who should be back?" I don't wish to sound evasive, but the short answer is always "it depends." The Celtics really need an upgrade in talent. If they get an upgrade or two, that will shift people from starting roles to backup roles and other people from backup roles right on out of the picture.
Pick just about any position and you can see how upgrading the starting spot would create a trickle down that just makes more sense. Center is the most obvious. Tyler Zeller is a solid contributor, but he's miscast as a starter. He would be a fantastic backup center, he just needs someone ahead of him in the rotation to make it make sense.
Then there's the power forward position. Brandon Bass (free agent), Jared Sullinger, and Kelly Olynyk could all start on a championship team if all the other pieces in place. It is like of like a commercial for a sugary cereal that ends with "part of this complete breakfast" (and it shows a healthy looking breakfast surrounding the box). If we had two stars, the Sully/Olynyk rotation at power forward would make sense going forward.
Still, at this point in their development, they would make a lot more sense coming off the bench. Thus if we got an upgrade at power forward, you could pick one of the two to be the backup and use the other in a package trade (either to do the upgrading at the 4 or to add pieces elsewhere). Bass I think would a fine fall back option if you had to use one of them in a trade and didn't have the pieces to upgrade the PF spot.
Jae Crowder has been a revelation at small forward. He's a glue guy that gives you a lot of things on the floor that help you win basketball games. He's still young so he's got the potential to be a lot more. Yet, right now he's not exactly a featured part of the offense. If you got a good scoring wing, Crowder would be an ideal fit next to (in the starting lineup) or backing up that scoring wing (while playing a lot of minutes along side him).
The guard positions are a little more tricky in that there are already 3 guys for 2 starting spots. Isaiah Thomas could be a starter, but he's just such an ideal fit as that 6th man scoring punch. Marcus Smart is likely going to be the team's long term solution at point, but he's still so young that it wouldn't hurt to have him coming off the bench. Avery Bradley is a versatile vet that can play combo guard and give you something positive on both ends of the court. Still, that skillset would be ideal as a 3rd guard. Somebody has to start so right now it is Smart and Bradley (for the most part) and I think long term that's the best fit going forward. Still, there's room there to upgrade those positions as well (given the right opportunity).
I think ideally I'd like to see the team add a starting center and a dynamic, scoring wing of some sort (preferably someone that can shoot from long range). That would shift things around so that guys better suited to backup or complementary starting roles would fall right into place. But the challenge is finding those guys and understanding what it will cost to get them (picks, players, or free agent cash). Basically, despite the positive end to the season and having a lot of good pieces to start with, Danny Ainge has his work cut out for him.