Replacing Rondo's off the charts intangibles and triple double potential (overrated or not no one was sneezing at Jason Kidd's triple doubles) is impossible, therefore if you're going to give him up you give him up for a PG who's an excellent shooter. Being able to score from the outside is just about the only way (if you don't get Paul or Deron Williams) to make up for the loss of dynamism that Rondo brings. That's why Conley is a possible fit, and DJ Augustin would be another potential fit. Not saying Conley and Augustin are not good PG's as well as shooters, but in terms of playmaking Rondo is probably in another league from those guys, not to mention rebounding, IQ, etc...
So Danny IMO would definitely take Rubio, who has a chance to replace some of the dynamism you lost with Rondo, and Rubio would eventually (don't know when) take over the starting role, giving you a sharpshooter PG off the bench. At the very worst you end with a decent-to-excellent trade asset down the road.
Bottom line, replacing Rondo (if Rubio can't start on day 1) is only really doable with an excellent shooter imo.
Danny wouldn't waste the #2 pick on any other prospect other than Rubio imo, unless Clifford Ray is drooling very persuasively to get his hands on Thabeet.