The way I see it, when we acquired Green, one thing that was pretty certain is that he was a fairly good third banana. Given that we have Rondo, this means that we really need to add one major piece rather than look for complementary, smaller pieces.
We're not shipping out Paul Pierce, and frankly, I don't think we have to. However, I do think that after Allen and Garnett are off the books next year, we're in a position to make a significant splash, and resign whoever of those two is interested to come back as a complementary player for the MLE. This will likely be Allen, since he looks like he can play until he's 40, has settled in the area, and generally is a good fit in almost any situation.
With all this in mind, whether they admit it or not, the Celtics are putting all their chips on Dwight Howard. He's the highest-profile FA next summer, addresses a position of need, and will make a make for a great core with Rondo, Green, and an aging Paul Pierce.
Sadly, there are a myriad of ways in which this can backfire, but I just can't look at the team and see how they're planning for anything else, short of a total blowup, which will likely just waste Rondo's prime.