sqiddster - families in families is the same as simply adding both families to one object type, so I don't think there needs to be a new feature for that. I'm not sure families with different plugin types is even possible, because the whole point of families is so you can write your events once. If you go to add an action for a family with both the Sprite and AJAX plugins in it, which actions do you show? What if you run an AJAX action on a Sprite instance or vice versa? etc...
As for the other use cases, I think the complexity problems I outlined with layer offsets means that it would be better to find a different way of solving those problems, without having to offset layers.
From a usability standpoint, allowing families in families is very different than saying one object can be in multiple families. Saying it's the same is like saying having families at all isn't needed because you can just copy and paste events for different objects. It makes maintenance so much easier.
As for your argument about Sprite and AJAX, obviously that would be totally useless. However when it comes to objects like Sprites, tiled backgrounds, text, etc, there are a LOT of common elements, and for something like a manual layer offset it would be very useful to be able to talk about all those objects at once. The sub-families feature also shows relevance too because I'm not going to want to manually add all the objects that could exist on that layer to this family manually.