I would like to see KG and Paul retire in Celtics green. I also think they wouldn't bring back much, due to their big contracts.
As crazy as this may sound, I think there is still a chance for another title, but as Roy said, the chances are small. I think the means of making this happen are quite narrow, and look something like this (everything I'm saying here has been mentioned elsewhere, but I think it's all accurate, and constitutes my best guess for a "formula for success"):
1) Acquire a legit center. This doesn't have to be an All-Star, but someone who is at least defensively competent and can block a few shots. My preference is Gortat, but even someone such as Mozgov could be enough.
2) Keep Avery Bradley at all costs. Defense wins titles, especially for this franchise, and AB is the best there is on the perimeter. Him, combined with a legit defensive C, will go a long way, I think.
3) Define roles once and for all. Make Jet part of the bench, and its primary offensive weapon, giving him freedom to freelance. Make Bass and Sully a permanent part of the bench. Give Green consistent minutes, and don't play him at the 4.