I'm a really big fan of the Ducks pattern where your action types are private to each reducer unless they're explicitly exported as an API. In my own code I don't even do that, instead exporting a function which operates on the state.
Hm, interesting. I like it. I'm also curious about how selectors come into play here as well (exported getter functions for shielding app from reducer state shape).
You might still need to place actions/selectors in their own directory since they're not necessarily specific to any reducer. I believe this is how sagas are organized, though.
https://github.com/erikras/ducks-modular-redux