We need to improve our starting lineup. Tucker and Frye are the most likely candidates to be replaced. The center position could use improvement too, but we have younger developing players in that spot, and it is much harder to find good centers.
IMO, Tucker is 50/50 on returning next year.
Our weakest position, by far, has been center but as you said, we have young players in development there. Our next weakest position is power forward, we need to either get a full season of the Channing Frye that played the first two months of this season or we need to find a power forward that can do all the things he does plus score and defend down low. I'm not convinced we can find someone with all those qualities so we may have to just settle for a stretch four that is more consistent than Frye.
As for PJ Tucker, I'm not sure we'll be able to find his equal let alone a player that impacts the game even more than him. I've often complained about the fact he's a dirt worker and that kind of player isn't supposed to make some of the ego/stupid mistakes PJ does but he just might be the heart and soul of this team.
Considering how much Tucker has improved his game from last season to this one I'm not willing to call him a finished product just yet. If he can learn to nail that baseline three, I see no reason he can't master that shot from elsewhere. If he's not asking for the moon I hope we keep him. It would be nice to find a small forward that beats him out of a starting position but he's too valuable to just let walk IMO.