P: Allow smart collections to reference other smart collections or library filter presets

This is a very good idea as long as the smart collection does not have to be in the same "tree" as the parent.

For example I would like to use the same definition of a smart collection across multiple publishing services.

Specifically I would like to see at the top of the smart collection definition dialog a drop down called "Inherits from" in which the user can specify an existing smart collection.

You mean as in built-in feature? or and addon?

Good idea (as an option):

I suppose it would be as simple as allowing inheritable collection criteria to be specified as part of the collection set definition, eh?

Another idea that might go well:

Have the combination criteria that is in play when multiple collections are selected be switchable between "union" (all included - the default now) and "intersect" (only if both criteria satisfied). This way a collection would not necessarily have to be a child to inherit from another collection, even if only a temporary thing.

Another idea:

Allow smart collections to be dragged and dropped on a smart collection to have criteria added. Maybe a drop modifier to dictate add vs. replace.

I think there is a feature to add other defined search criteria to new search criteria. But I can't find it.