They will never land a 'superstar' in his prime for Rondo. When the Hornets traded Paul, they first took Pau Gasol and change, then Eric Gordon and change over Rondo. That's an indication of his marginal market value, which certainly can't have gone up.
Maybe the goal in a trade, if you trade Rondo, should be a mature superstar viewed as starting the downside of his career, around the same age as Pierce/Garnett/Allen in 2008. Instead of trying to find someone young enough to be a Celtic for a decade, look for someone with the heart to be a Celtic for life.
Also, the Hornets would have received Lamar Odom, Kevin Martin, Luis Scola, Goran Dragic, and at least one first-round pick, not Pau Gasol, in the vetoed trade. And the Hornets didn't just get Gordon but a guy who was drafted #8 overall in Al-Farouq Aminu and a draft pick formerly belonging to the Timberwolves which ended up being high enough to draft Austin Rivers. A healthy Rondo, a young player with upside like Sullinger or Bradley, and a likely lottery pick would be the sort of package that could bring in a superstar big, but I'm not sure Sullinger or Bradley should be traded and the Celtics don't have that sort of pick anyways.