The fact that Rondo and Bradley were such a good pairing showed me that it's not as easy to predict who will go well with Rondo and who won't. Most people, myself included, did not think the two of them together would compliment eachother nearly as well as they do.
That said, I think Rondo mostly needs a big man who can contest the rim, rebound well, and QUICKLY outlet the ball so Rondo can push the ball. This type of player's defense can cover for the risks that Rondo takes on defense. For this reason I think Perkins and KG were good fits, but due to injury/age they aren't as much anymore. A big man who can also get out on transition would be great too.
Honestly, one of the first players that comes to mind is Josh Smith, but he just signed elsewhere. Dwight Howard too. Marc Gasol, if available in a year or two, could go well.
Another type of player who might fit well with Rondo is a deep post player like Zach Randolph. This would force some of the defense away from Rondo, enabling him to slash to the lane, etc.