Back in the 2008 Draft, I was hoping the C's could get Marreese Speighs, Jason Thompson, Javale McGee or DeAndre Jordan... all of them but Jordan were gone when Danny had the chance to pick. All 4 of them are promising big men in the league, so we can agree it was a mistake not to pick Jordan.
While I'd love the idea of getting any of them in free agency or via trade, I don't think the price will be fair, and I don't see any of them going to the Celtics. Danny needs to resign Nenad for reasonable money and years, so he can be a useful player and a good trading chip eventually.
This summer, if Semih can be 100% healthy and willing to play in the NBA, I would try to send back the pick to Cleveland to get him. Cleveland still wins the trade, by getting Gody, and I think Semih will be a good backup C in the regular season.
The other player I'd like to see in green is Greg Oden. If Portland extends the qualifying offer the Celtics could work a trade package for him. If Baby isn't returning, they could send him in a sign-and-trade plus the trade exception from Quis to match the money. Oden would make almost 8 mil, so it might be worth to take the risk, and if it pays out, you could resign him and Green to multi-year contracts.
The O'Neals situation will depend on the Celtics winning #18 this june, and their health. If they win the ring, I guess they return for more. If not, they could retire, so the Celtics will need bigs.