Kobe wasn't going to come to Boston and I don't think you could really take him at 6.
Assuming Bias was always going to die and thus you can't just keep him alive, there are three drafts that I think Boston's title chances might have been drastically altered with better drafting. 1989, 2001, and 2008. None of the other drafts would have had a shot at altering the title picture.
In 1989 if we would have added Tim Hardaway or Shawn Kemp to the aging vets and the rise of Lewis, I think Boston could have picked up a title or two in the early 90's before MJ and Pip really took off.
In 2001, had Boston come out of the draft with Joe Johnson, Zach Randolph, and Tony Parker to pair with PP, Toine, Battie, and Anderson, that team would have been very young and very good for a very long time.
In 2008, had we drafted Pekovic (though he wouldn't come over for awhile), Jordan, Asik, or even Chalmers instead of Giddens and then Dragic instead of Walker, we might have gotten another title or two out of the current group.