Among the features posed by this Qt user named Charley were whether items like C++14/C++17 support, Qt3D advancements, QML packaging work, cross-process signals/slots, or new modules would lead to a departure from Qt5 and ultimately to Qt6/Qt7. The mailing list discussion started here.
Developers responded that Qt3D continues advancing within Qt 5.x releases, QtRemoteObjects/Replicant is coming for cross-process signals/slots but hasn't been made public yet, etc.
Alan Alpert, a longtime Qt developer formerly working for Trolltech/Nokia and now at BlackBerry, explained his views on Qt 6:
Qt 6 break would be for breaking current features, not adding new ones. Odds are it's going to be driven from the QtQuick/QtQml side, as that's the part with the most stuff we want to break. So when we want to switch to QtQuick 3 and version 3 of the QML language, that's when I expect we'll start planning a Qt 6.While Alan doesn't carry the official view of the project, his statement is likely spot on based upon independent assessment and monitoring of their activity. Qt 6 will likely be some years away with Qt 5.0 having been released at the end of 2012 and new Qt5 updates adding exciting features until it comes time to breakage with Qt Quick 3 and QML3.
We're not there yet, but there are some things in QtQuick which are already scheduled for QtQuick 3 (when we get around to it). In particular the Window type needs to be broken out to be a QML-only wrapper type, based on QQuickItem, instead of exposing QQuickWindow directly. A bunch of properties also where we might want to change the default value, but that's just breaking stuff so it depends on how much of a break we want QtQuick 3 to be. We already know that we want it to be less of a break than QtQuick 1-2; since we don't expect to replace SceneGraph that could mean that QtQuick 3 and QtQuick 2 elements could co-exist on the same scene.
It's also expected that the new ModelViews would replace the current ModelViews in QtQuick 3. Given how we haven't even completed the research phase of the new ModelViews, you can see that QtQuick 3 is likely still years away.