The main architectural features we discussed in Barcelona are
Plone 5.2
- Zope 4, with ZServer by default
- Deprecation warning for Archetypes
- REST API
Plone 6.0
- Drop Archetypes from core
- Python 3
- Zope 4, with WSGI by default
There will certainly be other features, but these are the ones that really decide what the version number will be.