So the Nexus moniker for phones stops at 'Nexus 5'?
Seeing as how the number represents the screen size, I can see them calling the next one Nexus 5. But honestly I don't see them going to 6" anytime soon. Maybe the 6th generation might have a 5.2" screen at most.
So here is what I think will happen to the names:
2012: Nexus 4
2013: Nexus 5
2014: new Nexus 5 (2014)
2015: new Nexus 5 (2015)