For very high delta-V missions (eg outer solar system) the Falcon family really starts to suffer from that fact that its second stage is still using RP-1/LO2 rather than LH2/LO2 with a consequently lower ISP. I wondered just how complex it would be to use a Centaur (or an Ariane 4 ESC or eventually ACES) as a third stage (or in place of the second stage) on a Falcon 9 or Falcon Heavy launch. There are obvious political issues, but how hard are the technical issues and how much would you gain.
Much more difficult then it sounds. You would need the ability to fuel the upper stage, which would require major launch pad reorganization.
SpaceX's answer is to use the Raptor engine/ methalox, which won't be quite as good as LH2/LO2, but it should be pretty close. The extra thrust and less complex system should more than make up for any losses as well. This will come with the BFR, which is expected in a few years.