According to Taylor Wimberly, former editor for Android and Me and so far a great insider, Motorola will manufacture the next Nexus smartphone a.k.a. Nexus 5.
Here is his Google+ tip:
Motorola will release a Nexus smartphone in Q4 (that is not the Moto X).
He didn't elaborate further than that, so we can't give you any more details.
If this information is indeed true, then we expect Motorola and Google to announce the next Nexus smartphone in late fall.