Conor-Tony and Khabib-Barboza on the same card makes the most sense. Not only would it determine the next challenger, it would also provide a backup plan in case Conor or Tony is unable to fight.
None of this matters though, since Conor is probably fighting Nate next. And after that, it's probably on to another money fight, e.g., against Malignaggi, GSP, or Woodley.