While there is no public Odoo 19 source to be looked at, you can test the components of Odoo that have been released, as well as the current master branch which is the WIP for Odoo 19.0.
There is a summary of what has been shared more or less recently: https://www.odoo.com/forum/1/274049
Furthermore, with new versions, it usually goes similar to this:
- 18.0 [available in runbot]
- 18.1 (saas) [available in runbot]
- 18.2 (saas) [available in runbot]
- 18.3 (saas) [available in runbot]
Odoo 19.0 = Odoo 18.0 + 18.1 + 18.2 + 18.3 + 18.4 + master
Based on that structure, some educated guesses (not more, not less) can be made of what the initial release of the next version is aiming at. However, between x.4 and y.0, still, changes have the potential to go both ways.
runbot is at https://runbot.odoo.com