Here are a few points that come to mind:
→The Republicans have already said that they will continue their policy of obstructionism for any nominee Obama comes up with. Unless there is a recess appointment (which I see as unlikely) there will be no new appointee until after the inauguration in 2017.
→It is unlikely that Trump will get the Republican nomination, and it is likely that he will launch a third-party/independent campaign for President. This will effectively split the Republican vote, resulting in a landslide for the Clinton/Sanders (or maybe Sanders/Clinton?) ticket, as well as the so-called “down-ballot” candidates, sweeping a Democratic majority in to both houses on the coattails of the Presidential ticket.
→It has already been established that Obama is not interested in a transition from POTUS to SCOTUS, so he will probably not be the nominee. But given this scenario, I don’t think President Hillary (or even President Bernie) will have any trouble getting the Democratic-controlled congress to approve any nomination.