One outcome of the Plone 6 discussions at the conference in Ferrara is that we've decided to go ahead and create the Plone 6 branch.
The new 6.0 branch of buildout.coredev will use master branch of Products.CMFPlone.
The 5.2 branch of buildout.coredev will use the new 5.2.x branch of Products.CMFPlone.
At the moment, the primary feature of Plone 6 is the Dexterity-based site root object.
Please remember to port any relevant CMFPlone changes forward to that package's 6.0 branch.