Early reports surrounding the Nexus 6 from Motorola and Google stated that the phone would have a fingerprint sensor, but that is one of the things that didn't pan out in the end. Though, looking through the Android Open Source Project reveals the phone was going to support fingerprints and Android Lollipop was ready for that support as well.

The code removing support for fingerprint sensors can also be found, showing that for a period of time, a fingerprint sensor was truly going to be supported in the Nexus 6. Since progress was made with the fingerprint sensor feature before it was cancelled, we could definitely see a sensor in the next year or two.