I've returned recently ( for domain-specific reasons ) to sets of (opaque) god-classes with good interfaces when needed. No state in a god-class is visible except through the interface.

Since the domain requires a great deal of serialization, the interfaces are usually strings over that. In cases, it's even easier to just open a socket to the serialization interface.

So far, I'm able to pub/sub periodic data streams, but it'd be pretty easy to add "wake me when" operators and such.

It forces the use of one central table of names cross callbacks ( which can be grown dynamically ) but it's very, very nice to work with.

YMMV. The domain is instrumentation and industrial control, which just fits this pattern nicely. All use cases can and are specified as message sequences.