This topic is dealt with in Future Logic in Chapter 41, under the heading of Modalities of Subsumption.
http://www.thelogician.net/2_future_logic/2_chapter_41.htm
The existential import theory is rather limited because it only distinguishes between actual being and non-being, whereas in fact the way we use terms in practice is more complex, involving other possibilities.
Of course, an underlying issue is the nature of terms - i.e. concepts.
For that, look at my Phenomenology, chapter 4.
http://www.thelogician.net/2b_phenome_nology/2b_chapter_04.htm
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This essay explains why the existential import doctrine of modern logic is really trivial. It gives equal weight to empty terms, putting them on a par with realistic terms. Whereas in actual logical practice, once a term is classed as empty it ceases to have any significance in scientific and other realistic thinking. Read the essay!
