We already know that Google is planning to make Android's release cycle a little more predictable with quarterly maintenance updates. The first update is expected to include a developer preview, and it sounds like Google is planning to bump the version number up to 7.1. This comes from Google's crash and report filter in the developer console—it offers an option to filter for Android 7.1.
The highest we got on Marshmallow was 6.0.1. Moving to v7.1 from 7.0 could indicate a more substantial change than we expected for a maintenance release. This could bring the rumored API level 25, which we believe is being tested on the upcoming Nexus hardware.
Google said the developer preview of MR1 will be available this fall. That's when we're expecting to see new Nexus phones as well. There are some other reports of Android O being tested in addition to 7.1, but that seems unlikely. Probably just someone trolling.