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

The Any Filter plugin provides the ability to define a saved filter that references other saved filters and smart collections. Not quite as convenient as having it builtin to smart collections, but it may get the job done for you depending on your needs.

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.