If we are internalizing the league/container package, then I would suggest making some additional changes to it too:

• Make reflection a default feature instead of requiring an additional delegate container. Apart from maybe very small projects I doubt anyone would want to declare all definitions manually.
• Provide an API to add multiple definitions. Currently the only way to add multiple definitions together is to create a DefinitionAggregate instance and pass it as constructor argument. It would be a lot more convenient if there was for e.g. a Container::addDefinitions(array $definitions) method. Then app/plugins could even have config files of their own which return the required array.
• Update Definition::addArgument() to have and additional parameter for argument name. Currently if I have a service who's contructor has it's 1st argument as an object and 2nd argument as a scalar, there's no way to set a value for just the 2nd argument and let the 1st argument be handled by reflection.

These are few things that bugged me when using the container.

Make reflection a default feature instead of requiring an additional delegate container. Apart from maybe very small projects I doubt anyone would want to declare all definitions manually.

Currently I just added the ReflectionContainer by default as you can see here
So you would prefer to remove this kind of ReflectionContainer abstraction as a whole?

I always thought there were reasons why one want's to manually add services to a container instead of relying on Reflection.

Disabling reflection/autowiring should be an option but available by default IMO.