CakePHP 3.3 will add new large features and deprecations to the 3.x release series. It will be backwards compatible with the rest of 3.x

Add support for PSR-7 HTTP Messages. CakePHP will use a 'bridge' approach that will allow PSR7 middleware to be integrated into CakePHP applications. This bridge will be an optional layer in 3.3, but will be used by default in new applications. Controllers and Components will continue to get the current request object they get today.

A more detailed list of new features is documented in the migration guide