Okay, so if the Celtics flame out against the Heat (apologies for the pun) in 5-6 games, and the lockout doesn't completely prevent Danny from doing anything significant this summer, I'm going to assume that the rebuilding process will begin in earnest.
Here are my thoughts:
1) Doc Rivers will 'retire' as Celtics head coach. Lawrence Frank, Kevin McHale, or perhaps Armond Hill would be the most likely candidates, I think, to replace him.
2) Ray Allen, as the member of the Big 3 with the most value to a contender (solid 4th option shooter, can start or come off the bench), will be shopped around. Chicago seems like the team that could use him the most. The Knicks are unlikely to take on salary since they want Chris Paul. The Nets are unlikely for the same reason (they want DH12). I could see Dallas taking an interest, but they don't have much to offer. The Heat would probably love to have him, but they wouldn't have anything to trade, either. Perhaps a team like the Bucks would have some interest, since shooting is their biggest need and a year ago they were a borderline contender when their core players were healthy.
3) Danny will look at trading KG as well, either during the summer or before the trade deadline. As an expiring contract and a player who can still contribute in a diminished role, KG should have a fair amount of value to any number of teams. As with Ray, we'd be looking to take back expiring contracts, late first round / early second round draft picks, and maybe a fringe prospect.
4) Sign and trade options for Jeff Green and Glen Davis should be explored, as well. I'm not sure about the trade value of either of these players; it is likely that we can hold onto Jeff Green for another year for only a qualifying offer this summer. Giving him a full season to prove that he's worth keeping around in the rebuilding process might be a good idea.
The pros and cons of Glen Davis, however, are well known. He is not a player that can be a real asset to a rebuilding team. He should go be a role player on a playoff team somewhere, though I suspect he hopes to be a top option on a bad team. He should be signed and traded for anything we can get in return, or simply not signed at all if no trade can be worked out.
5) Keep Paul Pierce around. It'll maintain a team identity through the first couple years of the rebuilding process, and keep fans buying tickets to come to see their favorite hero in green from the last decade. He probably has the most trade value of any of the Big 3, but he has a couple years left on a hefty contract, so trading him would be complicated and we'd likely not be satisfied with what we'd get in return.
6) Shop Rajon Rondo around. Offer to combine him with Jeff Green or Glen Davis if it could net a good offer. If there's any chance at all that New Orleans would bite on an offer of Rondo + filler and picks for Chris Paul, you've got to do it. I really doubt they'd be interested in that, though; I think they can get more for CP3, who demonstrated in the first round of the playoffs that he can still carry a team with practically no talent around him.
In trading Rondo my hope would be to get back a young player with solid potential who won't make the team better in the short term but could be a very solid piece, either as part of the core or in a major trade, sometime in the near future.
Above all, my opinion is that the rebuilding process has to start, and it should start soon. Certainly nobody wants to see the Big 3 go, and it would be easier, in some ways, to hold on to this era and try to squeeze out a couple more 40-50 win seasons as a playoff team of moderate relevance. The prospect of returning to the mire of mediocrity in which this team was stuck for much of the 20 years before Garnett and Allen came to town is certainly not enticing.
But the best chance of transforming the old, past-their-prime Celtics of now into one of the next up-and-coming competitive teams in the league is to get the most value we can out of the players currently on our roster right now. As difficult as it may be, it makes sense to do it now because the window appears to be closed.