In the 2012-13 season, Ray Allen will be 37, KG 36, and Pierce 35. I don't think all 3 will still be playing, at least not with the celtics, and definately not at the same level they are at right now (although Ray is a freak of nature who doesn't seem to age). So I find it unrealistic to project a lineup of Rondo, Ray, Pierce, KG, and Dwight... However I must say, it is very fun to think about. I could see a lineup of:
Rondo
average - slightly above average SG
Pierce
random PF
Dwight
I know, a lot of question marks. I could be wrong, but I thought KG and Ray are off the books after next season. Will they retire? Will the sign one more extension with Boston? Or finish their careers elsewhere?